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COLLOID CHEMISTRY 


AN INTRODUCTION, WITH SOME 
PRACTICAL APPLICATIONS 


BY 
JEROME ALEXANDER, M.Sc., 


Consulting Chemist and Chemical Engineer, 
Past Chairman and Member, Commitiee on the Chemistry of Colloids 
(National Research Council); Fellow, American Association 
for the Advancement of Science; Member, Am. Inst. 
of Chemical Engineers, Am. Inst. Mining 
and Metallurgical Engineers, etc. 


ILLUSTRATED 


SECOND EDITION, REVISED AND ENLARGED 





NEW YORK 
D. VAN NOSTRAND COMPANY 


E1gHt WARREN STREET 


1924 


Copyright, 1919, by 
D. VAN NOSTRAND COMPANY 


Copyright, 1924, by 
D. VAN NOSTRAND COMPANY 


All rights reserved, including that of translation into 
foreign languages, including the Scandinavian 


PRINTED IN THE UNITED STATES OF AMERICA 


THE GETTY RESEARCH 
INSTITUTE LIBRARY 


PREFACE TO SECOND EDITION 





The favorable reception accorded the first edition of 
this little book has led the author to enlarge and ex- 
tend greatly this second edition, both in the theoretical 
and the technical sections, so that it may be used as 
an adjunct in teaching colloid chemistry, if not indeed 
as a text book. A large number of new practical 
applications have been introduced, and the effort has 
been made to develop the subject ina simple, coherent, 
and interesting manner, using, as far as possible, non- 
technical language and homely illustrations, so as to 
enable the reader’s interest to aid memory. 

The growing realization of the importance of colloid 
chemistry is evidenced by the fact, that, whereas the 
Decennial Index of Chemical Abstracts covering the 
years 1907-1916 contains only twelve columns of 
titles under Colloids, the 1922 Index alone contains 
five columns, and the 1923 Indexcontains four columns. 
These figures do not include many germane papers 
which are indexed under such headings as Adsorption, 
Diffusion, Coagulation, Gelatin, Gel, Sol, and a wide 
variety of physical, biological, and technical topics; 
but they do show that the colloidal zone is no longer 
“‘the world of neglected dimensions” as Wo. Ostwald 
once called it. 

Matter in the colloidal state has unique properties 
which necessitate a revision of some of our precon- 
ceived notions; and this fact cannot be avoided by 
attempting to alter definitions and the established 


lll 


1V PREFACE 


meaning of language, or nullified by a Procrustean 
effort to fit all experimental data to existing theories. 

To use a good old-fashioned term, we need natural 
philosophers, men whose view of the various fields of 
science is sufficiently broad and keen to enable them 
to see, understand, and correlate correctly apparently 
scattered facts in physics, chemistry, biology, tech- 
nology, and related branches. 

Science is not a sporting event. The true scientist’s 
sloganisnot ‘‘ May the best man win,”’ but rather ‘‘ May 
the truth prevail.” ; 

Colloid chemistry comes not to destroy, but to 
fulfill. It does not destroy or even replace the known 
facts of chemistry, physics, and other sciences, but 

‘draws attention to certain aspects of Nature which 


have often been overlooked. 
JEROME ALEXANDER 
50 East 41st Sr., 
N. Y. Crry, 
July 1, 1924 


PREFACE TO FIRST EDITION 





This little book is the result of an attempt to com- 
press within a very limited space the most important 
general properties of colloids, and some of the practical 
applications of colloid chemistry. Its object will be 
accomplished if it is helpful in extending the sphere of 
interest in this fascinating twilight zone between 


physics and chemistry. 
J. A, 
New YORE, 
Nov. 1, 1918 





7S 
i 


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aA 
_ 






TABLE OF CONTENTS 


Pee sCH ETO WSECOND FIDITION. . 0. 5.02056 eee ee ives cows 
Sreract TO ine? EDITION , .... . 2. oe ee be eee tae 
Cuapter 1—Introduction; Colloid Chemistry Defined; Sus- 
PEE A ONTELON «on. cue wk lilec od saute de pon aw ee 
CHAPTER 2—Material Units and the Forces Dominating Them; 
Divisibility of the So-called Elements; Chemical vs. 
Physical Forces; the Submicroscopic Structure of 
Matter; Hydrogen Ion Concentration; Homogeneity and 
Heterogeneity; Phases; Interfacial Anomalies; Residual 
Affinities; A Simple Principle Underlying the Colloidal 

State; The Zone of Maximum Colloidality; The Relation 

of Colloidal to Other Forces; Solution vs. Colloidal 
Solution; The Nature of Adhesion; Surface Forces in 
Grinding or Pulverizing; The “Colloid Mill”.......... 
CuapteR 3—Classification of Colloids..................... 
CuapTeR 4—Consequences of Subdivision................. 
CuHapTeR 5—The Ultramicroscope................0200000- 
CuHaptTeR 6—General Properties of Colloids; Colloidal Pro- 
~ tection; Gold Number; Double or Plural Protection; 
Autoprotection; Cumulative Protection; Dialysis; 
Ultrafiltration; Diffusion; Electric Charge and Migra- 

tion; Pectization and Peptization; Viscosity.......... 
Cuaprer 7—Practical Applications of Colloid Chemistry; 
Astronomy; Meteorology; Smokes, Fogs and Dusts 
(Aerosols); Perfumes; Geology; Mineralogy; Gems... . 
CuHapTER 8—Practical Applications (cont.); Agriculture; 
Clays; Ceramics and Refractories; Flotation.......... 
CuarptTerR 9—Practical Applications (cont.); Dyeing; Shower 
Proofing Fabrics; Nitrocellulose and its Products; 
Celluloid; Explosives; Paints, Pigments, Varnishes; 
a LE a en ene nee Manis fur atansen ae 
Cuapter 10—Practical Applications (cont.); Soaps; Lubri- 
cation; Coal; Colloidal Fuel; Petroleum; Asphalt; 

UP OE PAY LOUOOLIOLOCS oor. oce-c4)x sy siete s styay sags ewe 
Cuapter 11—Practical Applications (cont.); Filtration; 
Sewage Disposal; Photography; Brewing; Tanning; 
Te ELULNAINS Ghai are c'se v6 hh wie se blag, Wa aN Wnty seats a ew ahs 


7-25 


26-28 


29-32 


33-36 


37-54 


55-72 


73-85 


86-97 


98-110 


111-123 


Vlil TABLE OF CONTENTS 


CuapterR 12—Practical Applications (cont.); Foods and their 
Preparation; Baking; Milk; Ice Cream; Confectionery; 
Gelatin ‘and ‘Glue. 2)... 4... acs vs ok be eee 124-136 
CuapreR 13—Practical Applications (cont.); Glasses; Metals 
and Alloys; The Time Factor; Iron and Steel; Steel; 
Stepped Transformation in Steel; Standard Heat 
Treatment Terms; Tin-Lead Alloys; Zinc-Copper Alloys 
(Brass); Bronze; Amorphous vs. Colloidal Theory; 
Electrodeposition of Metals; Boiler Scale; Cement, | 
Mortar and. Plaster.....40. 6e0.4 604. oo ee 137-164 
CuapTer 14—Practical Applications (cont.); Chemical An- 
alysis; Pharmacy and Therapeutics; Antiseptics and 
Bacteriology; Biology and Medicine; Enzymes; Cytol- 
ogy; Growth; Evolution; Physiology and Pathology; 
Capillary Circulation; Psychiatry; Anaphylaxis and Im- 
munity; Healing of Wounds; Digestion; Absorption, 
Secretion, Excretion; Diagnosis; Chemo-Therapy and 
Colloid Therapy; Serum Therapy; Plants; Bio-Electric 


Currents... 0006 o sia cas 0 ee ge 165-194 
BIBLIOGRAPHY... ..4.000000 lees eecss pe 195 
GILOBBARY 4.4.4 06 ec. 0 eeu een culesce sa oe ee 197 
AUTHOR INDEX. .0.0 54404 des cls cee eet ee 199 


COLLOID CHEMISTRY 


—_—_—— 


CHAPTER 1 


INTRODUCTION 


Although many facts and principles concerning col- 
loids have from time immemorial been known and 
utilized empirically, the scientific foundation of modern 
colloid chemistry was laid by an Englishman, Thomas 
Graham, F.R.S., Master of the Mint. In two basic 
papers on this subject, the first entitled ‘‘ Liquid Dif- 
fusion Applied to Analysis,” read before the Royal 
Society of London, June 13, 1861, the second entitled 
‘‘On the Properties of Colloidal Silicic Acid and other 
Analogous Colloidal Substances,” published in the 
Proceedings of the Royal Society, June 16, 1864, Gra- 
ham pointed out the essential facts regarding colloids 
and the colloidal condition, and established much of 
the nomenclature in use at the present day. In the 
first of these papers Graham says: ‘‘The property of 
volatility, possessed in various degrees by so many 
substances, affords invaluable means of separation, 
as is seen in the ever-recurring processes of evaporation 
and distillation. So similar in character to volatility is 
the diffusive power possessed by all liquid substances, 
that we may fairly reckon upon a class of analogous 
analytical resources to arise from it. The range also 
in the degree of diffusive mobility exhibited by different 
substances appears to be as wide as the scale of vapor 
tensions. Thus hydrate of potash may be said to pos- 

: 1 


2 COLLOID CHEMISTRY 


sess double the velocity of diffusion of sulphate of 
potash, and sulphate of potash again double the veloci- 
ty of sugar, alcohol and sulphate of magnesia. But 
the substances named belong all, as regards diffusion, 
to the more ‘‘volatile’ class. The comparatively 
‘‘fixed’’ class, as regards diffusion, is represented by a 
different order of chemical substances, marked out by 
the absence of the power to erystallize, which are slow 
in the extreme. Among the latter are hydrated silicic 
acid, hydrated alumina and other metallic peroxids of 
the aluminous class, when they exist in the soluble 
form; with starch, dextrin and the gums, caramel, 
tannin, albumen, gelatin, vegetable and animal extrac- 
tive matters. Low diffusibility is not the only property 
which the bodies last enumerated possess in common. 
They are distinguished by the gelatinous character of 
their hydrates. Although often largely soluble in 
water, they are held in solution by a most feeble force. 
They appear singularly inert in the capacity of acids 
and bases, and in all the ordinary chemical relations. - 
But, on the other hand, their peculiar physical aggre- 
gation with the chemical indifference referred to ap- 
pears to be required in substances that can intervene 
in the organic processes of life. The plastic elements 
of the animal body are found in this class. As gelatin 
appears to be its type, it is proposed to designate sub- 
stances of this class as colloids, and to speak of their 
peculiar form of aggregation as the colloidal condition 
of matter. Opposed to the colloidal is the crystalline 
condition. Substances affecting the latter form will 
be classed as crystalloids. The distinction is no doubt 
one of intimate molecular constitution. 

‘‘ Although chemically inert in the ordinary sense, 


INTRODUCTION 3 


colloids possess a compensating activity of their own, 
arising out of their physical properties. While the 
rigidity of the crystalline structure shuts out external 
impressions, the softness of the gelatinous colloid par- 
takes of fluidity, and enables the colloid to become a 
medium for liquid diffusion, like water itself. The 
same penetrability appears to take the form.of cemen- 
tation in such colloids as can exist at high temperature. 
Hence a wide sensibility on the part of colloids to 
external agents. Another and eminently character- 
istic quality of colloids is their mutability. Their 
existence is a continued metastasis. A colloid may 
be compared in this respect to water, while existing 
liquid at a temperature under its usual freezing-point, 
or to a supersaturated saline solution. Fluid colloids 
appear to have always a pectous modification; and 
they often pass under the slightest influences from the 
first to the second condition. The solution of hydrated 
silicic acid, for instance, is easily obtained in a state of 
purity, but it cannot be preserved. It may remain 
fluid for days or weeks in a sealed tube, but is sure to 
gelatinize and become insoluble at last. Nor does the 
change of this colloid appear to stop at that point. 
For the mineral forms of silicic acid deposited from 
water, such as flint, are often found to have passed, 
during the geological ages of their existence, from the 
vitreous or colloidal into the crystalline condition. 
(H. Rose.) The colloidal is, in fact, a dynamical state 
of matter, the crystalloidal being the statical con- 
dition. The colloid possesses Energia. It may be 
looked upon as the probable primary source of the 
force appearing in the phenomena of vitality. To the 
gradual manner in which colloidal changes take place 


4. COLLOID CHEMISTRY 


(for they always demand time as an element) may the 
characteristic protraction of chemico-organic changes 
also be referred. .. . 

‘It may perhaps be allowed to me to apply the con- 
venient term dialysis to the method of separation by 
diffusion through a septum of gelatinous matter. The 
most suitable of all substances for the dialytic septum 
appears to be the commercial material known as vege- 
table parchment, or parchment-paper. .. .” 

At the beginning of the second paper above referred 
to, Graham states: “‘The prevalent notions respecting 
solubility have been derived chiefly from observationson 
crystalline salts, and are very imperfectly applicable to 
the class of colloidal solutions.” From this it may be 
seen that Graham appreciated the fact that all the laws 
of crystalloidal solutions could not beapplied to colloidal 
solutions. In the case of erystalloidal solutions the 
dissolved substance is present in a state of molecular 
subdivision, and, according to the ionization theory, 
is in many cases dissociated into ions. With colloidal 
solutions, on the other hand, we have a lesser degree 
of subdivision, and the particles in solution are larger 
and more cumbersome. As Graham remarked, ‘‘The 
inquiry suggests itself whether the colloid molecule 
may not be constituted by the grouping together of a 
number of smaller crystalloid molecules, and whether 
the basis of colloidality may not really be this com- 
posite character of the molecule.” This is to-day the 
idea generally accepted. 


Colloid Chemistry Defined 


Colloid chemistry deals with the behavior and 
properties of matter in the colloidal condition, which, 


INTRODUCTION 5 


as we now know, means a certain very fine state of 
subdivision. While there are no sharp limitations to ' 
the size of particles in colloidal dispersions, it may ina 
general way be stated that their sphere begins with 
dimensions somewhat smaller than a wave length of 
light, and extends downward well into dimensions 
which theory ascribes to the molecules of crystalloids. 
(See Table IT, p. 27.) 


Suspension vs. Solution 


With the aid of the ultramicroscope, which renders 
visible particles approaching in minuteness molecular 
dimensions, Zsigmondy has shown that there is no sharp 
line of demarcation between suspensions and colloidal 
solutions, but that with increasing fineness in the sub- 
division of the dissolved substance, there is a progres- 
sive change in the properties of the resulting fluids, 
the influence of gravity gradually yielding to that of 
the electric charge of particles, of surface tension and 
of other forms of energy. ‘Thus in the case of metallic 
gold, subdivisions whose particles are 1 uw and over act 
as real suspensions and deposit their gold, whereas 
much finer subdivisions (60 uu and under) exhibit 
all the properties of metal hydrosols or colloidal solu- 
tions. In the ultramicroscope the coarser subdivisions 
show the well-known Brownian movement, which 
greatly increases as the particles become smaller, until 
at the present limit of ultramicroscopic visibility 
(about 5 wu) it becomes enormous both in speed and 
amplitude. | 

On the other hand, there is no sharp distinction be- 
tween colloidal and crystalloidal solutions, but as the 
particles in solution become smaller and smaller, the 

2 


6 COLLOID CHEMISTRY 


optical heterogeneity decreases correspondingly, finally 
vanishing as molecular dimensions are approached.* 
That even crystalloid solutions are not in a strict sense 
homogeneous, is indicated by an experiment of van 
Calcar and Lobry de Bruyn (Rec. Trav. chim. Pays-Bas, 
1904, 23, 218), who caused the crystallization of a 
considerable part of saturated crystalloid solutions at 
the periphery of a rapidly rotating centrifuge. 


*In an article entitled “‘Pedetic Motion in Relation to Colloidal 
Solutions” published in Chemical News, 1892, Vol. 65, p. 90, William 
Ramsay, Ph.D., F.R.S. (afterward Sir William Ramsay), clearly ex- 
pressed this view in the following words: ‘I am disposed to conclude 
that solution is nothing but subdivision and admixture, owing to attrac- 
tions between solvent and dissolved substance accompanied by pedetic 
motion; that the true osmotic pressure has, probably, never been 
measured; and that a continuous passage can be traced between 
visible particles in suspension and matter in solution; that, in the 
words of the old adage, Natura nthil fit per saltum.” 


CHAPTER 2 


MATERIAL UNITS AND THE FORCES DOMINATING THEM 


Next to accurate observation and due allowance for 
all influential factors involved, perhaps nothing is more 
important in scientific matters than the mental separa- 
tion of seasoned facts from the theories designed to 
systematize and explain them. For facts are hard, 
stubborn things that survive the theories whose down- 
fall they may cause. 

A theory must interminably run the gauntlet of the 
whole far-flung tribe of scientists, and may be stricken 
down after long years of apparent safety. Einstein’s 
Theory of Relativity has been checked by astronomers 
and physicists in such diverse fields as the deflection of 
alpha particles shot out from radium, the deviation of 
starlight by the sun, and the precession of the axis of 
the orbit of the planet Mercury. How convenient it 
would be if a scientist, when confronted with facts that 
run counter to the very theories he has been taught to 
revere, could only say, as Ko-Ko remarked to Pooh- 
Bah :—‘‘ Come over here where the Lord High Treas- 
urer can’t hear us!” 


Divisibility of So-called Elements 


Among the disconcerting discoveries of recent times 
may be mentioned those connected with radio-activ- 
ity, which eventually led to the demonstration that 
electricity consists of discrete particles, and that our 
supposedly infrangible elements are complexes of 
positive and negative electrons, and that some of these 

7 


8 COLLOID CHEMISTRY 


complexes are breaking up spontaneously and uncon- 
trollably, while others may be shattered by the terrific 
impact of alpha particles moving at the rate of 10,000 
miles or more per second.* 

Then the basic assumption of Dalton’s atomic 
theory fell, when T. W. Richards, J. J. Thompson, 
Aston, Dempster, Harkins and others showed that 
with many of the elements, the atoms are not all 
alike, but have different atomic weights. 

It is startling to learn, for example, that lithium 
consists of a mixture of atoms having atomic weights 
of 6 and 7, chlorine of atoms having atomic weights 
of 35 and 37, and that krypton has six isotopes, as 
they are called. In an address before the Chem- 
ical Society (London) Aston remarked:—‘Though 
as a chemist I view with some dismay the possi- 
bility of eighteen different mercuric chlorides, as a 
physicist it is a great relief to find that nature 
employs at least approximately standard bricks 
in her operations of element building.” And, to 
the endless disgust of small boys, Harkins has 
pointed out the possibility of 63 different kinds of 
calomel. 

Some of our other new discoveries cast doubt upon 
what were heretofore accepted as facts. Thus the 
recognition of vitamines and certain salts as food essen- 


*To give an idea of the minuteness of an electron, Prof. R. A. 
Millikan states that if the entire population of Chicago, estimated at 
2,500,000 people, were to begin to count the number of negative elec- 
trons passing as an electric current through an ordinary incandescent 
lamp in one second, and-were to count continuously day and night at 
the rate of two per second, it would take them about 20,000 years to 
finish the count. In the highest vacuum we can produce, the number 
of residual molecules is so great that in each cubic inch there is a mole- 
cule for every inhabitant of the earth. 


MATERIAL UNITS 9 


tials has necessitated the revision of conclusions drawn 
from some previous experiments on food values, which 
in some cases showed simply the minimum quantities of 
food needed to supply the necessary amount of vita- 
mines or salts. The incompleteness of mere calorie- 
fat-protein-carbohydrate specifications is now well 
recognized. 


Chemical vs. Physical Forces 


We make free use of the expressions ‘‘physical 
mixture” and “chemical compound”’ with the full 
confidence that we know exactly what we are talking 
about. But when we attempt to define these terms 
exactly, we appreciate the truth of Aristotle’s remark 
that to frame an exact definition requires complete 
knowledge. 

The difference between chemical and physical 
attraction is explained in many elementary text books 
by taking the example of iron filings and flowers of 
sulphur. From their physical mixture the iron may 
be separated magnetically or the sulphur dissolved 
out by carbon bisulphide; and the two ingredients 
may be separately identified in the microscope. If 
they are heated, however, the non-magnetic chemical 
compound FeS is formed, from which CS. extracts 
no 8. Furthermore, while Fe and S may be physi- 
cally mixed in any desired proportions, in the chemi- 
cal compound FeS they combine in the definite pro- 
portions of 56 parts by weight of Fe to 32 parts by 
weight of S, and any excess of S may be dissolved out 
by CS. 

This fixes homogeneity and closeness of union in 
definite proportions by weight, as criteria of chemical 
combination. 


10 COLLOID CHEMISTRY 


But the transition between chemical and physical 
forces is not so sharp as our definitions demand; and 
it is in the transition zone that there appear the 
phenomena encountered in that extremely fine state 
of subdivision or dispersion known as the colloidal 
condition, where combination depends not upon the 
total mass involved, but upon the total free or active 
surface. 


The Sub-microscopic Structure of Matter 

A whole series of complexities underlie even the 
smallest particle of matter visible in the most powerful 
- microscope. Negative electrons, and protons or posi- 
tive electrons are the only material units not at present 
known to be complex. While the number of steps in 
this series may vary in individual cases, in many 
instances the following degrees of aggregation may be 
definitely traced. 





Order | Order of 
Material Unit of Com- Mode of Examination 
Size* plexity 


Protons see 107 up 0? Positive ray and particles 
from radium 
Hlectsons 203 ei ee 2x107 uy 0? Electric field, while float- 
ing on ultramicroscopic 
particles 
ATOMS yon ore eRe 0.1-2.0 uy 1 X-ray spectrometer 
Molecules... oar 0.5-5.0 uy 2 X-ray spectrometer 
Molecular Groups..... 1-10 pp 3 Diffusion and Ultrami- 
croscope 
Primary Colloidal Par- 
hicles: 253. Cee 2-20 pp 4 Diffusion and Ultrami- 
croscope 
Secondary Colloidal 
Particles yc e..eee 5-100 wy 5 Diffusion and Ultrami- 
: croscope 
Microscopically Resolv- 
able Particles....... over 6 Microscope 
250 pp 
Visible Particles....... fas 7 Eye 
M 


* Diameter. 


MATERIAL UNITS 11 


Taking Shapleigh’s estimate of the diameter of the 
galaxy of 300,000 lighty ears = approximately 2.5 & 10?! 
cm., we see that in an exponential range of 37 * we may 
pass from the smallest known material unit, the pro- 
ton, to the outermost confines of the universe. This 
indicates the extent of the variations in real values 
represented by the py exponential figures so often 
used to express the effective acidity or alkalinity of 
colloidal solutions. When using Sérenson’s conven- 
lent Py symbolism we must remember that it is an 
inverse exponential function. 


Hydrogen Ion Concentration 


The expression p, demands some explanation. It 
is a convenient way to express the reaction of any 
fluid, but is dangerous because it does not do this 
directly; for it is an inverse logarithmic function, 
deprived of its minus sign, as will be seen directly. 

Pure water dissociates slightly according to the 


equation 
H,O = H* + OH- 


the reaction being reversible. It has been found that 
at 22° C. the concentration of hydrogen ions in pure 


water equals moles per liter, a fact usually 


1 
10,000,000 
expressed by the symbolism C,+ (concentration of 
hydrogen ions) equals 10~’ moles per liter. Since for 
each H* ion in water there is an OH™ ion, Cog- 
= 10~’ under the same conditions. 

* That is the proton has a diameter of the order of 107* cm. and 


the galaxy a diameter of the order of 10 cm., the exponential spread 
being 37. 


12 COLLOID CHEMISTRY 


Now, for convenience, Sérensen proposed to dis- 
regard the minus sign, and to use simply the numerical 
value of the exponent of 10 to express the reaction 
represented by the corresponding C,+. The following 
table* will make this clear: 


Degree of Equivalent 
normality Crt Pu value 
HCl 1.0 8.0 x107 0.10 
0.1 8.4 x10 1.07 
0.01 9.5 x10°3 2.02 
0.001 9.7 x10-4 3.01 
0.0001 9.3-x40°% 4.01 
Acetic acid 1.0 4.3 x1073 2.37 
0.1 16 x103 2.87 
0.01 4.3 x10 3.37 
0.001 16 x10 3.87 
Caustic soda 1.0 0.90 x 10-44 14.05 
0.1 0.86x1073 13.07 
0.01 0.76 x 10-2 12.12 
0.001 0.74 x 10-4 11.13 


This table shows that the lower the p, value, the 
greater the effective reaction of acidity, which is some- 
thing quite different from the total acidity. Thus 
equal volumes of normal hydrochloric acid and of 
normal acetic acid will neutralize equivalent volumes 
of normal alkali, but the normal acetic acid has a 
much lower degree of acidity or hydrogen ion con- 
centration, and therefore a higher p, value. 

Not only does the p, value run opposite the H ion 
concentration, but the variations are exponential or 
logarithmic, not arithmetical. The step between Deo 
and py 6 is vastly less than the step between Dao 
and py 4, as may be seen from the following table: 

*“*Die Wasserstoffionen Concentration,” by Prof. Lenor Michaelis. 


MATERIAL UNITS 13 


Number of times H (or OH) ion concen- 


Pu value tration exceeds that of pure water 
1 1,000,000 
2 100,000 
3 10,000 
4 1,000 
5 100 acid side 
6 10 
vf 0 pure water 
8 10 
9 100 
10 1,000 alkaline side 
11 10,000 
12 100,000 
13 1,000,000 


Homogeneity and Heterogeneity—Phases 


Lewis and Randall (‘‘Thermodynamics,”’ p. 9) de- 
scribe a homogeneous system as one in which there are 
no apparent surfaces of discontinuity; and a hetero- 
geneous system as one consisting of two or more dis- 
tinct homogeneous regions or phases, which appear to 
be separated from each other by surfaces of discon- 
tinuity. The celebrated Phase Rule of J. Willard 
Gibbs [Trans. Conn. Acad. Sci., 3, 228 (1876)] is that 
the number of degrees of freedom (F’) of a system as a 
whole (in other words the number of independent 
variables upon which it depends) equals the number 
of variables essential to determine the state of the 
individual phases, minus the sum of the phases out- 
side the first. 

Thus in a system of n phases whose individual 
states are controlled by x variables, 


F=x2-—-(n-—-1)=24-n+1 | 
But as Zsigmondy pointed out (Colloids and the 


14 COLLOID CHEMISTRY 


Ultramicroscope, p. 3) and as the above table show- 
ing the serial complexity of matter indicates, the 
practical distinction between homogeneity and hetero- 
geneity depends entirely upon the refinement of our 
means of observation. In many cases we assume 
phases to be homogeneous to simplify our calcula- 
tions; but when a substance is finely subdivided or 
dispersed, the influence of specific surface (exterior or 
rind surface) becomes an important factor, the phases 
neglected or submerged for our convenience urgently 
demand consideration, and the number n in Gibbs’ 
equation must be increased. 


Interfacial Anomalies 


In all free surfaces the exterior molecules occupy a 
position quite different from the interior molecules, 
which are on all sides surrounded by their own kind. 
Part of the attractive forces of the surface molecules 
are directed inward, part toward adjacent surface 
molecules (thus forming a surface skin), and a residual 
unsatisfied portion outward. This residual affinity 
may be demonstrated by cleaving a piece of mica 
(muscovite) and instantly putting the new surfaces 
together. They exhibit considerable cohesion, al- 
though this represents but a fraction of the original 
attractive force, and is soon lost by further adsorption 
of atmospheric constituents. A fresh iron surface 
absorbs atmospheric constituents so quickly that, 
although a piece of cast iron broken under mercury is 
amalgamated at its new surfaces, the same iron if 
broken above the mercury and instantly dropped in, is 
not amalgamated at all. (P. W. Bridgman.) 


MATERIAL UNITS 15 


Residual Affinities 


If the attractive forces existing between atoms were 
entirely balanced or satisfied by their chemical com- 
bination, then every chemical compound would be- 
have as a perfect gas so far as concerns the factor a in 
the equation of van der Waals. But in all chemical 
compounds exist residual attractions or stray fields of 
force which exert a.controlling influence on what are 
ordinarily termed the physical properties of the com- 
pound—its state (gaseous, liquid or solid), its co- 
hesion, solubility, boiling point, conductivity for heat 
and electricity, dielectric constant, etc. This residual 
attraction is responsible for the phenomena we call 
adhesion and adsorption, and its range of effective 
action (of the order of 10-* cm.) is much less than the 
diameter of a molecule. 

Langmuir says that molecules usually orient them- 
selves in definite ways in the surface layer, since they 
are held there by forces acting between the surface and 
particular atoms or atomic groups in the adsorbed 
molecule. This accounts for the influence of chemical 
composition on adsorption commented on by Bechhold 
and others. Harkins (J. Am. Chem. Soc. 1920, 42, 
706) points out that in organic compounds this 
orientation depends upon the dissymmetry of the 
molecule. ‘‘An entirely symmetrical molecule (or 
atom in the case of monatomic liquids) would not 
orient at all, but such a molecule or atom does not 
exist. Molecules as symmetrical as those of the short 
chain saturated paraffins, carbon tetrachloride, etc., 
would not be expected to show such evidence of orien- 
tation as those molecules which may be considered as 
heavily loaded, from the standpoint of the stray 


16 COLLOID CHEMISTRY 


electro-magnetic field, at one end of the molecule and 
very light, in the same sense, at the other.” 


A Simple Principle Underlying the Colloidal State 


The anomalous properties exhibited by matter in 
colloidal dispersion, as we follow it beyond the limit 
of microscopic resolvability to molecular dispersion, 
may be understood by considering two properties— 
(1) specific surface and (2) kinetic activity. 

Specific surface (exterior or rind surface per gram) 
will vary considerably with the actual shape of the 
particles. If we assume an average spherical shape 
and plot specific surface against particle size, we 
obtain an hyperbola. For with infinitely large par- 
ticles the free surface per gram approaches zero, and 
with infinitely small particles the free surface per gram 
approaches infinity; and the areas of spheres vary 
inversely as the squares of their radii. Therefore the 
curve is asymptotic to both axes and varies as ad; 
that is, it is hyperbolic. 

The mean velocity of translation * of particles is like- 
wise subject to great variations; chemical nature, phys- 
ical structure, viscosity of the dispersing medium, 
temperature; incidental ions, solutes, or impurities, all 
exercise their effects. But with the same substance, 
under the same conditions, the kinetic motion, begin- 
ning as the well-known Brownian motion at the limit of 
microscopic resolvability, increases at first slowly and 
then with gradually accelerating rapidity as subdivision 
proceeds; so that the curve of kinetic motion, plotted 
for convenience of comparison on the same axes as that 


* There is a kinetic motion of revolution as well as of translation, 
the observed translatory kinetic motion being the difference. 


MATERIAL UNITS 17 


of specific surface, soon rises rather abruptly, and cuts 
the latter (see Fig. 1). 





Increasing particle size 
AB = free surface per gram; CD = kinetic motion curve; EF = 
rise and fall of colloidal characteristics. 


Fig. 1.—Relation between kinetic activity and specific surface. 


The Zone of Maximum Colloidality 


In following the gradual dispersion of a substance 
from gross visible particles all the way down to true 
molecular subdivision, it is obvious that in the colloidal 
zone there is a transition from a state where the kinetic 
activity is small as compared to the specific surface to 
a state where the kinetic activity is large as compared 
to the specific surface. In the heart of the colloidal 
zone there is usually observable an indefinite portion 


18 COLLOID CHEMISTRY 


of the curve which has been termed the zone of maxi- 
mum colloidality (J. Alexander, J. Am. Chem. Soc. 
43, 434 (1920), wherein colloidal properties, such as 
viscosity, are most marked. 

The zone of maximum colloidality appears in a 
wide variety of substances—in steel (J. Alexander), in 
duralumin (Merica, Waltenberg & Scott), in dyeing 
(R. Auerbach). In his H. M. Howe Lecture (before 
the Am. Inst. Min. & Met. Engr. 1924) Prof. Albert 
Sauveur stated facts which disclose its existence in 
heated steel, which showed maximum strength over 
a certain temperature range. On adding water to 
acetone solutions of nitrocellulose, F. Sproxton (3d 
Report on Colloids, etc., Brit. Assoc. Adv. Science, 
1920) found that the viscosity rose to a maximum and 
then fell. Sulfur at 150° has a viscosity of 8; at 
about 200° it rises to over 50,000, and at 400° it drops 
to 150. Viscose in ageing passes through a maximum 
of viscosity. 

We may therefore pass upward into the colloidal 
state as molecules aggregate or become larger, or pass 
downward into the colloidal state as coarse particles 
are more finely ground or dispersed. Thus Martin H. 
Fischer showed that in taking sodium salts of the 
fatty acids beginning with sodium formate and passing 
through the acetate, propionate, etc., up to the stearate 
and oleate, we find that the water holding capacities 
of the salts increases steadily.* His results may be 
tabulated as follows: 

*L. Lascary [Koll. Z., 34, 73 (1924)] found that the effect of sodium 


soaps on surface tension increases with molecular weight to sodium 
myristate, above which it decreases. 


MATERIAL UNITS 19 


No. of 
Carbon 
Atoms 
1...Sodium formate..... molecular Gisperaon 
“ acetate..... i 
Soeae | propionate.. c 
a7. Dutyrate.... a si 
5 ‘* valerate... . ou 3 
6... ‘“  caproate....shows signs of holding water 
Bee 9 capryiate ...1 mol. Bue 2 jelly with 250 c.c. of water 
Tee eaprate..... Lair i 500 c.c. 
Peewee S leurate;. ... lige? ot aero at ee: 774000 C.c: 
fame eeesinvristate,,.1 ‘ ‘ 123.000 ec. 
Preeeomoniunitate, .1 *  ‘ 90000 ce. 
eee ecmarparate ..1 ‘ ee ae 24 OO) 6.0; 
ieee ptearate..... Aye" a gene ee OOO Cra, 
mieeeeeearacnnate...1 ‘ ° “  . 37.000 c.c. 


On the other hand, Victor Lehner reduced silica to 
colloidal solution by Poncied grinding; and colloidal 
gold may be produced by the Bredig or the Svedberg 
method of electrical spattering, as well as by the 
Faraday-Zsigmondy method of reducing gold chloride. 

The above facts justify Svedberg’s classification of 
methods for the formation of colloids into (a) aggre- 
gation methods, (b) dispersion methods. 


The Relation of Colloidal to Other Forces 


Mendeléef in his suggestive paper, entitled “A 
Chemical Conception of the Ether,” intimated that 
gravitation might be explained on the basis of etherial 
impact acting from all sides; for from this assumption 
the Newtonian laws of gravitation may be easily de- 
veloped. This is evident from the following con- 
siderations: 

Assuming that the ether is a subtle gas whose par- 
ticles move with approximately the speed of light, and 
that mass represents the ability of a substance to reflect 


20 COLLOID CHEMISTRY 


the motion of ether particles, then any body, if alone 
in the universe, would be struck equally on all sides 
by the ether particles and if at rest would remain 
at rest, but if in motion would tend to maintain 
its rate of motion. This is Newton’s first law; but it is 
obvious that if the motion of the body is large as 
compared with the motion of ether particles, then the 
moving body will appreciably increase in mass; for it 
will anticipate the blows of ether particles coming from 
the direction of its motion, and receive a diminished 
impact from those coming from the opposite direction 
(relativity effect). 

As soon as we introduce a second body into our 
calculation, then each body shadows the other in 
proportion to its mass or ether-stopping capacity. 
This means an excess of ether pressure on the un- 
shadowed side, so that the bodies attract each other 
directly as their masses. But since the areas of 
spheres vary inversely as the squares of their radii, it 
is obvious that a body removed to twice the distance 
will have only 1/, the shadowing effect; at three times 
the distance only 1/, the shadowing effect, etc. Thatis, 
the attraction between the bodies varies directly as 
their respective masses and inversely as the square of the 
distance between them (Newton’s second law). 

EK. Cunningham (“Relativity and the Electron 
Theory”) has shown that a material ether is not incon- 
sistent with relativity, and the objective view of gravi- 
tation is much more appealing than recourse to “fields 
of force’ without a material substratum, although, as 
Herbert Spencer pointed out, explanations of this kind 
simply transfer the enigma of Nature one step further 
back. 


MATERIAL UNITS 21 


M. Le Sage in 1784 had expounded the same theory 
of gravitation, and Prof. 8. P. Langley (inventor of the 
airplane) had a translation of Le Sage’s work pub- 
lished by the Smithsonian Institution. It is pos- 
sible that the nature of chemical attraction may 
be explained as a gravitational force modified because 
the reacting units are so close together that their 
size, shape and internal structure become material 
factors. We seem to be working toward the view 
maintained by Faraday and no doubt by many oth- 
ers, that there is one ultimate kind of force. 

P. E. Wells (J. Wash. Acad. Sci. 1919, 9, 361) 
suggests the following classification of forces: 

1. Electronic Forces—Maintain positive nucleus, and 
negative or valence electrons in equilibrium as a 
single system. 

2. Atomic Forces—Maintain two or more atoms in 
equilibrium as a single system. 

3. Molecular Forces—Maintain two or more mole- 
cules in equilibrium as a single system. 

4. Molar Forces—Maintain two or more masses in 
equilibrium as a single system. 

Electronic forces are thus responsible for atoms; 
atomic forces, for their chemical combination into 
molecules; molecular forces, for most physico-chemical 
and colloidal phenomena; and molar forces, for or- 
dinary physical and astronomical phenomena. ‘‘ Each 
group of forces,’”’ says Wells, ‘‘may be regarded as the 
residual fields of force remaining unsaturated in the 
smaller systems constituting the components of the 
system under consideration. . . . Molecular systems 
have lost so much of their discreteness that combina- 
tions of molecules do not follow the laws of definite 

3 


22 COLLOID CHEMISTRY 


and multiple proportions. In such phenomena as 
molecular association and surface structure, the dis- 
creteness of atomic constitution begins to give place to 
statistical continuity. Moreover, in these phenomena 
the force are relatively so weak that molecules are not 
usually regarded as permanently grouped together.” 

Thus adsorption, molecular association, and con- 
densation would be considered as molecular phenom- 
ena; and while polymerization may start that way, 
in it atomic forces predominate. 

In reading over the numerous practical applications 
of colloid chemistry we will later consider, it must be 
remembered, then, that most of the reactions lack 
that preciseness indicated by the present meaning of 
the term ‘‘chemical compound.” Slight deviations 
from previous conditions may involve a material 
difference in results, and until we understand and can 
give proper weight to all the underlying factors, many 
of our successful methods will remain ‘‘cooking 
recipes.”’ Colloid chemical research is letting in a 
flood of light upon many reactions not amenable 
to the ordinary stoichiometric laws of chemistry, 
though in some cases they may closely approach 
these laws. 


Solution vs. Colloidal Solution 


When the attraction of the molecules of a liquid for 
those of a solid exceeds the attraction of the molecules 
of the solid for each other, then the solid is dissolved 
or peptized by the liquid. If the dispersion is pro- 
found enough, the solute may go into true or crystal- 
loidal solution, but if the molecules of the solute cling 
to each other to a considerable extent, they may form 


MATERIAL UNITS 23 


groups of colloidal dimensions, and there results a 
colloidal solution.* 

The degree of dispersion in which a substance exists 
in a solution naturally affects its kinetic activity, its 
speed of diffusion, and even its ability to diffuse. Thus 
sodium stearate is not a good protector nor even a good 
detergent until it is dissolved in sufficiently heated 
water; and in tanning and dyeing the degree of dis- 
persion governs fixation capacity and speed. In the 
case of a kinetically balanced equilibrium, as with 
blood sugar, difference in diffusion speed may raise or 
lower the percentage of dextrose in the blood. 

An interesting case is the action of selenium oxy- 
chloride on barium sulphate. As Lehner discovered, 
this remarkable solvent, which dissolves such diverse 
substances as rubber and gelatin, converts the highly 
insoluble barium sulphate into a colloidal jelly. Molec- 
ular groups having a great tendency to dissolve are 
sometimes able to drag into solution with them at- 
tached groups which would otherwise be insoluble. 
Thus many aniline dyes and even oils are sulphonated 
in order to make them ‘“‘soluble”’ in water, where they 
usually form colloidal dispersions. 


The Nature of Adhesion 


Our principal adhesives, such as gums and glues, 
are substances which because of the makeup of their 
molecules possess powerful residual fields of force and 
are therefore capable of attaching themselves to other 
substances possessing residual fields of the opposite 
charge. They must besides have sufficient residual 

* Indeed, with most solutes and with most solvents, too, there is a 


certain degree of molecular association, as it is called. Liquid water 
is mainly dihydrol (H20)s. 


24 COLLOID CHEMISTRY 


force to cling powerfully to adjacent molecules of their 
own kind. Thomas Graham long ago remarked that 
colloids as a rule adhere better to each other than to 
crystalloids. One reason probably is that colloids 
possess a plurality of residual fields which may have 
different charges at different points. They are also 
highly polar. 

Adhesion is purely a matter of surfaces. Glue will 
stick paper to wood, but not to paraffin or to wood 
having a paraffined surface. It is true that glue will 
hold paper to paraffin or to bright tin until the adhesive 
dries; then the powerful attraction of the glue par- 
ticles for each other as they are dehydrated breaks 
the weak bond between the glue and the paraffin or 
metal, although it is unable to break the bond with 
the paper to which the other face of the glue layer 
clings tightly. 


Surface Forces in Grinding or Pulverizing 


It is not an easy or inexpensive operation to dry 
grind most substances below 200 mesh. As subdivi- 
sion proceeds, the total free surface increases enor- 
mously, and apart from the inherent difficulty of 
breaking up a very fine particle, the tendency of the 
fragments to reunite or ‘‘cake up” begins to assert 
itself. The finer the particles the greater this tend- 
ency, and it is increased by pressure. It is to a large 
measure overcome by wet grinding, because the ad- 
sorption of fluid or solutes at the new surfaces tends to 
prevent their reunion. 


MATERIAL UNITS 25 


The ‘ Colloid ” Mill 


This is essentially a high speed disintegrator ar- 
ranged for wet grinding, and the patents of Block, 
Plauson and China relate chiefly to mechanical details. 
A readily absorbable substance (a deflocculator or pro- 
tective colloid) is added to the liquid in the mill, and 
stabilizes the colloidal dispersion as it is formed. 

Block’s machine, has a very high speed rotor ec- 
centrically placed in a casing of circular cross section. 
The high peripheral speed of the rotor forces the mate- 
rial to be pulverized, practically under pressure, into 
the space between the rotor and the casing, where most 
of the atomization occurs, the material to a large extent 
grinding against itself. 

‘The mill can be used to make emulsions, and col- 
loidal solutions of cellulose and highly concentrated 
colloidal mercury have been made with it. It is said 
to be used in the soap industry for speeding up saponi- 
fication; in the dairy industry for homogenizing milk 
and cream; in producing colors, inks, rubber goods, 
etc. No doubt it will produce many desirable results, 
but since high speed takes power and means wear, the 
commercial advantages of the mill remain to be dem- 
onstrated. (See e.g. 8th Report on Progress of Ap- 
plied Chemistry, Soc. Chem. Ind., 1923, page 340). 


CHAPTER 3 


CLASSIFICATION OF COLLOIDS 


The broadest classification of colloids is that of 
Wolfgang Ostwald (Koll. Zettschr., Vol. 1, page 291), 
who grouped them according to the physical state 
(gaseous, liquid or solid) of the subdivided substance 
(dispersed phase), and of the medium in which the 
particles of the subdivided substance are distributed 
(dispersion medium).* Table I below shows the nine 
resulting groups and gives some instances of each. 

Ostwald’s classification, however, is more theoretical 








TABLE I 
Dispersed | Dispersion 
phase. | medium. Example. 

Gas Gas io oe: No example, since gases are miscible in all 
proportions. 

Caste es Liquid): . Fine foam, gas in beer. 

(288 ss Solid... ... Gaseous inclusions in minerals (meer- 
schaum, pumice), hydrogen in iron, oxy- 
gen in silver. 

Liquid...... Gascon seis pra s fog, clouds, gases at critical 
state 

Liquid..... Liquid..... Emulsions of oil in water, cream, colloidal 
water in chloroform. 

AS0UId ea DOlid.. wie Mercury in ointments, water in paraffin 
wax, liquid inclusions in minerals. 

BOG t ea ce Gap ciaaes Cosmic dust, smoke, condensing vapors 
(ammonium chlorid). 

GUE trys eons Liquid..... Colloidal gold, colloidal sodium chlorid, 
colloidal ice in chloroform. 

PSOUL Arc oeie Ol? Srnec Solid solutions, colloidal gold in ruby glass, 


coloring matter in gems. 


* G. Bredig proposed to call colloids “microheterogeneous systems.” 
W. Ostwald called them “dispersed heterogeneous systems,’ which 
expression was contracted by P. P. von Weimarn into the term 
“dispersoids.” 

+ A. Einstein, “Turbidity near the Critical State,” Ann. Phys., 33, 
1275 (1910). 

26 


TABLE II 

















4 


Sous) en -6- 1 6 ns 

















1p 

















ve lengths 


| 


: 


f Light. 





joe a py a 








[ Gold Sus- 
J ension C 


I 
elleidal—Sohations 





0.1p. 




















2 eee 
Reversible “Hydrosols. | Trreversibie Hydrosols; 























MN 








Tp 

















V1°0 
BI 








nit 
OD S¢ 





il 








| 





























Or 











alt | cit in} 


182 
S|TBDIOL 




















ii 





































































































Crystalloid=Solutions ; 


SSS ————————— 
0.14 2 ———and-vaactares= 5 


























Classification of Colloidal Solutions 


according to the size of the particles contained in them and 
according to their behavior upon desiccation. 





Microscopic Field. 


Field. 


Ultramicroscopic 








CLASSIFICATION OF COLLOIDS aT | 


than practical, for the properties of colloids are 
dependent mainly upon the specific nature of the 
dispersed substance and its degree of subdivision. 
Following Hardy, Zsigmondy divided colloids into two 
classes, the reversible and irreversible; the former 
redissolve after desiccation at ordinary temperatures, 
whereas the latter do not. 

Table II, taken from Zsigmondy,* illustrates this 
classification, and shows how colloids having the same 
particle size or degree of subdivision may nevertheless 
act quite differently because of specific differences in 
the nature of the dispersed substances. 

With the reversible colloids (gelatin, gum arabic, 
albumen), there is a more intimate union between the 
two phases; in fact it is probable that with them we 
have really a mixture of (1) a dispersed phase of water 
subdivided in the solid, with (2) a dispersing phase of 
the solid finely subdivided in water. The reversible 
colloids are therefore called emulsoids and the irrevers- 
ible colloids suspensoids. Colloids of the reversible 
type are also said to be hydrophile or lyophile, while the 
irreversible colloids are hydrophobe or lyophobe.t 

No sharp line is to be drawn, however, for besides 
intermediate or transition cases between the two 
classes, there may be recognized two groups of irrevers- 
ible colloids, roughly defined by their behavior upon 
concentration: 

First: The completely irreversible, which coagulate 
while still quite dilute and separate sharply from the 
solvent with the formation of a pulverulent precipitate 
rather than a gel (i.e., pure colloidal metals). Chem- 

* “Colloids and the Ultramicroscope,” J. Wiley & Son, Inc. (Trans- 
lation by J. Alexander.) 

+ Hydrophile = water-loving; hydrophobe = water-hating. Lyo- 
phile = solution-loving; lyophobe = solution-hating. 


28 COLLOID CHEMISTRY 


ical or electrical energy is needed to bring them back 
again into colloidal solution. 

Second: The incompletely reversible which, when 
quite concentrated, form a gel that may be easily 
redissolved or peptisized by comparatively small 
amounts of reagents, unless the evaporation has pro- 
ceeded too far (i.e., colloidal stannic acid). 


CHAPTER 4 


CONSEQUENCES OF SUBDIVISION 


As the subdivision of a substance proceeds, the area 
of its effective surface increases enormously, as may 
be seen from the following Table III adapted from 
Ostwald. Consequently surface forces, such as adsorp- 
tion, capillarity and surface tension, become enor- 
mously magnified and of primary importance. Further- 
more, the so-called radius of molecular attraction 
(p = 50 wy) is well within the colloidal field, so that 
the specific attractive forces of the particles also enter 
as a controlling factor. In fact, before substances can 
unite chemically their particles must be first brought 
into proper subdivision and proximity,* by solution, 
fusion, ionization or even by mere pressure, as was 
demonstrated by W.-Spring, who caused fine dry 
powders to combine chemically by high pressure. If 
the degree of subdivision is not profound enough to 
permit of the combination of isolated atoms or ions 
with each other, chemical combination in the strict 
sense may not occur, but there may be produced ‘‘ad- 
sorption compounds” resulting from the union of 
atomic or ionic mobs in indefinite or nonstoichiometric 
proportions, under the influence of more or less 
modified chemical forces. The combination of arse- 


nious acid and ferric oxid which Bunsen regarded as a 
* It is a striking fact that absolutely dry sodium is not attacked by 
absolutely dry chlorin. M. Raffo and A. Pieroni observed that colloidal 
sulphur reduced silver salts energetically, whereas even fine precipitated 
sulphur did not form silver sulphide in the cold, and did so only partially 
upon boiling. 
29 


COLLOID CHEMISTRY 


30 


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CONSEQUENCES OF SUBDIVISION ok: 


basic ferric arsenite, 4 Fe.O3;, As.O3, 5 H.O, has been 
shown by Biltz and Behre to be an adsorption com- 
pound; and Zsigmondy proved ‘‘purple of Cassius’’ 
to be an adsorption compound of colloidal gold and 
colloidal stannic acid by actually synthesizing it by 
mixing the two separate colloidal solutions. Zsig- 
mondy’s proof has been further confirmed by A. 
Huber (Physikal. Zeit. (1924), 25, 45) who examined 
the purple of Cassius with the X-ray spectrometer 
and found that none of the gold is in chemical com- 
bination. | 

The effect of increasing subdivision upon the par- 
ticles in colloidal solutions is illustrated in Table IV, 
adapted from Zsigmondy. Tables V and VI were 
prepared by Zsigmondy to illustrate visually the rela- 
tion of the sizes of colloidal particles to well-known 
microscopic objects on the one hand and to the theo- 
retical sizes of molecules on the other. 


COLLOID CHEMISTRY 


32 





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TABLE V 
LINEAR MAGNIFICATION 1: 10,000 





A. Human blood corpuscles (diameter 7.5 u, thickness 1.6 p). 

B. Fragment of rice starch granule (according to v. Héhnel) 3-8 p. 

C. Particles in a kaolinsuspension. 

#. Anthrax bacillus (length 4-15 uw, width about 1 yz). 

F. Cocci (diameter about 0.5-1 u, rarely 2 u). 

f,g,h. Particles of colloidal gold solutions Auy3,, Atg:, AUo7 (0.006-0.015 x). 
i, k, l. Particles from settled gold suspensions (0.075-0.2 yw). 


ee 78 


TABLE VI 
LINEAR MAGNIFICATION 1: 1,000,000 





t 


a-d. — Hypothetical Molecular Dimensions 


Hydrogen molecule — dia. 0.1 yy. 

Alcohol molecule — dia. 0.5 up. 

Chloroform molecule — dia. 0.8 uy. 

Molecule of soluble starch — dia. about 5 pp. 


e-h.— Gold Particles in Colloidal Gold Solutions 


Gold particle in Auj. (too small to determine). 
ss of c's Sa bowtel eau 
“e “es 6é oe i 4é 3.0 wy. 
“TAU, 1 0 ihe 
oe A Ueto eee 
Aug, “ 15 pps 


Gold particle in settled gold suspension, 


66 66 6é 


CHAPTER 5 


THE ULTRAMICROSCOPE 


As this instrument revolutionized colloid research, 
a brief description of it is essential. 

It is a matter of every-day experience that the unseen 
motes and dust particles in the air become visible in a 
beam of bright light, especially against a dark ground, 
and in this simple fact lies the principle of the ultra- 
microscope. 

Faraday and later Tyndall made use of a convergent 
beam of light to demonstrate the optical inhomogeneity 
of solutions; for in fluids not optically clear, the path 
of the beam becomes more or less distinctly visible, 
because of the light scattered by the particles present. 
In this manner can be recognized much smaller 
quantities of matter than by spectrum analysis—in 
fact less than 10-8 mg. (1/10,000,000) of metallic gold 
can thus be detected with the naked eye. 

Prof. Richard Zsigmondy while experimenting with 
colloidal solutions conceived the idea of examining this 
light cone microscopically. His preliminary experi- 
ments having demonstrated that he could thus see the 
individual particles in various hydrosols, he sought the 
assistance of Dr. H. Siedentopf, scientific director of 
the Zeiss factory, in Jena, where was produced the 
first efficient ultramicroscope. 

The ultramicroscope consists essentially of a com- 
pound microscope arranged for examining in a dark 
field an intense convergent beam of light cast within 
or upon the substance under examination. The light 

33 


34. COLLOID CHEMISTRY 


seen by the eye represents, therefore, the light dif- 
fracted, scattered or reflected upward by the substance 
or by particles within it. 

If within a thin beam of light from a projection 
lantern we scatter successively powders of different 
substances in various degrees of fineness (mica ground 
to pass 60, 100 and 160 mesh; lampblack; powdered 
oxid of zinc; flake and powdered graphite), some of 
them will produce only a homogeneous illumination 
of the beam in which no isolated particles can be seen, 
whereas with others, the individual particles are dis- 
tinctly visible. 

Passing the beam through a beaker of distilled 
water, nothing can be seen; but upon the addition of a 
few drops of colloidal gold solution, which appears 
quite clear to transmitted light, the path of the beam 
through the fluid immediately becomes visible. ‘This 
Tyndall effect,* as it is called, might be considered a 
criterion of colloidal solution were it not that very 
minute traces of colloidal impurities can produce it and 
it is often exhibited by solutions generally regarded as 
crystalloidal—those of many dyestuffs for example; 
furthermore with increasing fineness of subdivision the 
Tyndall effect decreases, disappearing as molecular 
dimensions are approached. 

Just as in the cosmic field our most powerful 
telescopes fail to resolve the fixed stars, which are 
nevertheless visible as points of light of varying bril- 
liancy, so, too, in the ultramicroscopic field, we can see 
particles much smaller than the resolving power of the 
microscope (that is, smaller than a wave length of 
light) provided only that they diffract sufficient light 

* Also known as the Faraday-Tyndall effect. 


THE ULTRAMICROSCOPE 35 


to affect the retina. Based upon the experience of 
astronomers we may be able greatly to increase the 
sensitiveness of the ultramicroscope by fortifying the 
eye, so to speak, with the photographic plate, using at 
the same time tropical sunlight or ultraviolet * light for 
illumination. 

In the original form of the ultramicroscope, as 
perfected by Siedentopf and Zsigmondy, which is the 
one best adapted for the examination of transparent 
solids, a side illumination is effected by a microscope 
objective with micrometer movements, which throws 
an intense but minute conical beam of light into the 
fluid contained in a little cell having quartz windows 
at the side and top. Above this cell a compound 
microscope is adjusted vertically, so that the nar- 
rowest part of the light cone occupies the center of the 
focal plane. If the fluid under examination is optically 
clear or if it contains particles so small that they cannot 
diffract sufficient light to create a visual impression, 
the light cone cannot be seen. If enough light is dif- 
fracted, the light cone becomes visible, being homo- 
geneous if the particles are too small or too close 
together to be individually seen, and heterogeneous if 
the particles can be individually distinguished. Parti- 
cles or dimensions beyond the resolving power of the 
microscope (about 14 y) are for brevity termed wltram- 
crons. Ultramicrons that can individually be made 
visible are called submicrons (or hypomicrons) while 
those so small that they produce an unresolvable light 
- cone are termed amicrons. 

Knowing the percentage of gold present in a col- 
loidal gold solution and assuming a certain specific 

* This has recently been done by T. Svedberg. 


36 COLLOID CHEMISTRY 


gravity and uniform shape for the gold particles, the 
average size and mass of a single particle of colloidal 
gold can be calculated, if the number present in a given 
volume be first counted. In this manner Zsigmondy 
has shown that the smallest particles of colloidal gold 
which can be individually distinguished with bright 
sunlight are approximately 5 wy in diameter, that is, 
five millionths of a millimeter; still smaller particles 
exist but they produce only an unresolvable light cone. 
Magnified 1,000,000 times such a tiny gold particle 
would be about 14 inch in diameter, while a human red 
blood corpuscle would be about 25 feet across, and a 
hydrogen molecule a speck barely visible. The gold 
particles in the unresolvable light cone must therefore 
closely approach molecular dimensions. In fact, by 
allowing amicrons to grow into visibility in a suitable 
solution and then counting them, Zsigmondy has 
recently shown that some of the particles of colloidal 
gold have a mass of 1-5.10~'* mg., indicating a size of 
1.7to3 pup. 

Various other types of ultramicroscopes, mainly modi- 
fications of dark field illumination, have been developed 
by Cotton and Mouton, Ignatowski (made by Leitz), 
Siedentopf (cardioid condenser, made by Zeiss) and 
others, and besides being useful in examining colloidal 
solutions, they have enabled pathologists to see and 
discover ultramicroscopic bacteria (spirochetes, infan- 
tile paralysis). 

Bausch & Lomb Optical Co. of Rochester, N. Y., 
are now producing a useful ultramicroscope. 


CHAPTER 6 


GENERAL PROPERTIES OF COLLOIDS 


The optical properties of colloids and their simula- 
tion of chemical compounds have been already referred 
to. The other general properties of colloids may be 
considered under the following headings: 


. Colloidal Protection. 

. Dialysis, Ultrafiltration and Diffusion. 

. Electric Charge and Migration. 

. Pectization (Coagulation) and Peptization. 
. Viscosity. 


oR WD eS 


Colloidal Protection 


Without an understanding of the underlying prin- 
ciples, isolated facts have long lain like individual 
gems scattered or even lost in the wide fields of human 
experience. A theory is necessary to string them 
together, each in its proper place, to form a beautiful 
necklace; or rather, a theory to serve as a matrix that 
forms of them a cutting drill, with which we may 
bore into the hard rocks of the still unknown. 

Continually, throughout chemical literature, the use 
of protective substances crops up, only to be buried 
again or to be hidden in the oublietie of some rule-of- 
thumb formula. The aurum potabile of the alchemist 
was made by reducing gold chloride in the presence of 
ethereal oils. Then followed the use of solutions of 
tin which stabilized the gold sol by the stannic oxide 
simultaneously produced, giving the rich purplish red 
solution known as the purple of Cassius. In 1794 

4 37 


38 COLLOID CHEMISTRY 


colloidal gold produced in situ was known as a dye 
for silk; in 1821 egg-white, isinglass, and starch were 
used as protectors for gold sols; and the Lehrbuch of 
Berzelius (1844) contains several recipes for producing 
gold sols of various shades. In 1856 Faraday * reports 
the discovery of jelly (evidently isinglass or gelatin) 
as a protector to colloidal gold; and, finally, Zsig- 
mondy (1898), then unaware of the preceding work, 
rediscovered gold sols.+ 

A most important contribution of Zsigmondy, how- 
ever, was his demonstration that stable though highly 
sensitive gold sols could be made without the use of 
protectors provided the presence of coagulators was 
avoided, and also the proof that these sensitive sols 
could be stabilized by the addition of gelatin, gum, 
and the like. Then only did the full import of 
colloidal protection begin to dawn and the use of the 
principle, as such, begin to be understood and de- 
veloped. Zsigmondy also synthesized the purple of 
Cassius by simply mixing the pure colloids of gold and 
stannic acid, thus proving that here the stannic acid 
takes the part of a protector. 

Von Meyer and Lottermoser { had just previously — 
recognized protective action as such, calling attention 
to the fact, long utilized by practical photographers, 
that albumin stabilized silver sols. Lottermoser § 
later said that ‘“‘on the addition of very stable colloids, 
as albumin, gelatin, agar, or gum arabic, to a silver 
sol, no precipitation is caused by electrolytes until the 
stable colloid is coagulated. The less stable silver sol 


* Phil. Trans., 1857, p. 145. 

Tt ‘Colloids and the Ultramicroscope,” 1909. . 
tJ. prakt. Chem., 56 (1897), 241. 

§ “Anorganische Kolloide,”’ 1901, p. 50. 


GENERAL PROPERTIES OF COLLOIDS 39 


is thus protected against the electrolyte by the more 
stable colloid; it becomes more like the latter in its 
behavior.” 

Gold Number 


With his pure but sensitive ruby-red gold sols (gold 
content about 0.005 to 0.006 per cent), Zsigmondy 
then established the relative protective value of a 
number of protective substances. The ‘gold figure,” 
compiled from the results of Zsigmondy and Schryver 
(see below), indicates the number of milligrams of 
protector which just fail to prevent the coagulative 
color change from red to violet of 10 cc. of the colloidal 
gold solution upon the addition of 1 cc. of 10 per cent 
sodium chloride solution. It must be remembered 
that the relative protective values were determined 
with gold, and while they are generally maintained, 
they may be different with other substances, especially 
those that coagulate the protector or adsorb it poorly. 
A substance fixed by adsorption often does not behave 
as it does when free, so that stability and protective 
power are not necessarily parallel functions. 


SUBSTANCE GoL_p NUMBER 
Gelatin 0.005 to 0.01 
Russian glue 0.005 to 0.01 
Isinglass 0.01 to 0.02 
Casein (in ammonia) 0.01 
Egg-globulin 0.02 to 0.05 
Ovomucoid 0.04 to 0.08 
Glycoprotein ~~ 0.05.t0 0:1 
Amorphous egg-albumin 0.03 to 0.06 
Crystallized egg-albumin 2.0 to 8.0 
Fresh egg-white 0.08 to 0.15 
Gum arabic 0.5 to 4.0 
Gum tragacanth 2.0 + 
Dextrin 6.0 to 20.0 
Wheat starch 5.0 + 


Potato starch 25.0 + 


40 COLLOID CHEMISTRY 


Sodium oleate 0.4 to 1.0 
Sodium stearate at 100 degrees 0.01 
Sodium stearate at 60 degrees 10.0 
Deutero-albumose re) 
Cane sugar ee) 
Urea 00 
Stannic acid sol (old) 0 


Zsigmondy, following W. B. Hardy, classified colloid 
sols into two broad classes, based on their behavior 
on desiccation—the reversible or resoluble, and the 
irreversible or irresoluble sols. These correspond 
roughly with the groups hydrophile-hydrophobe 
(Freundlich), lyophile-lyophobe (Perrin), and sus- 
pensoid-emulsoid (Wo. Ostwald). Although some 
salts (citrates, sulfocyanates) may act as protectors, 
protection is generally accomplished by adding a 
reversible or emulsoid colloid to an irreversible one, 
which thereupon acquires reversible properties—that 
is, it becomes insensitive to electrolytes, redissolves 
after desiccation (at any temperature that does not 
render the protector insoluble), and passes through 
ultrafilters that would otherwise hold it back. 

The generally accepted explanation of this phe- 
nomenon is that advanced by Bechhold,* that the 
protector is adsorbed at the free surfaces of the pro- 
tected particle. Zsigmondy supports this view, al- 
though he was unable to detect ultramicroscopically 
any sign of the adsorbed layer or any diminution in 
the Brownian motion. This is not surprising, as the 
protecting layer is probably only one or two molecules 
thick. Another possibility is that the protector, 
following its adsorption, may change the net charge 
of the particles. 

* Z. phys. Chem., 48 (1904), 385. 


GENERAL PROPERTIES OF COLLOIDS 41 


Experimentally, it is a most surprising fact that 
certain minimal quantities of protectors actually 
sensitize instead of protect, and may themselves even 
produce flocculation. Thus, according to Bechhold,* 
0.0003 to 0.0001 parts of gelatin per million will 
flocculate gold sols or oil emulsions. The work of 
J. Billiter + indicates that this is due to the fact that 
in such cases the minute amount of added protector 
brings the other colloid to or near the isoelectric 
point, where, as Hardy has shown, all colloids are 
especially susceptible to coagulation. 

From what has been said it is evident that although, 
in general, oppositely charged colloids precipitate each 
other, if an excess of a positive protective reversible 
(emulsoid) sol is quickly added to a negative ir- 
reversible (suspensoid) sol, the protector is adsorbed 
and protects before precipitation can occur. If, how- 
ever, only a very small quantity of the protector is 
used, or, what amounts practically to the same thing, 
if the protector be added very slowly, precipitation 
may occur before protection can be established—that 
is, precipitation may depend upon the speed of mixing 
as well as on the relative proportions mixed. This 
gives us also an explanation of the curious ‘‘ zones of 
precipitation”’ investigated by Neisser and Friedmann, 
H. Bechhold, Teague and Buxton, A. Lottermoser, 
and J. Biltz. 

It is not at all strange that the behavior of a sol 
should be in large measure controlled by the nature 
of the outer layer or skin of its particles. Adhesion, 
as is well known, depends upon the nature of the out- 
side layers, and so in fact does chemical action; for, 


* “Colloids in Biology and Medicine,” translated by J. G. M. Bullowa. 
1 Z. phys. Chem., 51 (1905), 142. 


42 COLLOID CHEMISTRY 


while the nucleus of an elemental atom is mainly re- 
sponsible for its atomic weight, its chemical behavior 
‘is governed by its outer ring or layer of electrons. 

It would be a long task to discuss the numerous 
cases where colloids are used, wittingly or unwittingly, 
in scientific, medical, or technical practice, as pro- 
tectors, deflocculators, and emulsifiers; or where they 
must be removed because their presence is detri- 
mental. Problems of this kind abound in such widely 
diverse fields as photography, metallurgy, brewing, 
rubber, paper, glass, filtration, cooking, cement, 
agriculture, tanning, paints, pharmacy, biology, etc. 
Colloidal sols exist everywhere in the organism, and 
no reaction takes place there without being influenced 
by their presence. 

To illustrate the working of protection, compare 
precipitates of silver chloride and tin or lead ‘‘ trees” 
made in the presence and in the absence of gelatin, 
or divide a lead acetate solution into three parts, 
adding a little hot gelatin solution to the last. The 
first will give with hydrochloric acid a curdy pre- 
cipitate of lead chloride; the second will give with 
sodium chloride (which is less highly ionized than 
HCl) a turbid lead chloride sol; while the gelatin- 
containing solution will, preliminarily at least, show 
no visible turbidity with sodium chloride. 

Simple colloidal protection has long been known. 
Let us now consider some newer aspects of this re- 
markable phenomenon—namely, double or plural 
protection, autoprotection, and cumulative protection. 


GENERAL PROPERTIES OF COLLOIDS 43 


Double or Plural Protection 


In 1908, while experimenting with milk, it seemed 
that casein might be an adsorption product containing 
protein and calcium salts. Accordingly, an attempt 
was made to produce a colloidal precipitate of calcium 
phosphate in the presence of such protectors as gelatin 
and gum arabic. The protector was added first to 
the sodium phosphate alone and then to the calcium 
chloride alone; but in neither case could a precipitate 
be obtained which approached casein in fineness or 
stability. 

Upon following mentally the course of the formation 
of colloidal calcium phosphate in the body, the idea 
was conceived that, since in the organism all reacting 
fluids hold protectors, perhaps the result could be 
obtained by adding some protector to both of the 
reacting solutions. ‘The experiment was at once tried, 
and yielded, with gelatin as protector, a colloidal 
calcium phosphate which could be precipitated by 
both acid and rennin. Upon emulsifying some olive 
oil in the mixture, a stable artificial milk was obtained. 
Thus was developed the principle of double or plural 
protection, for which a U. 8. patent was secured. 

The literature contains several instances where this 
principle has been used in a more or less empirical 
manner. Thus, Carey Lea produced some of his 
colloidal silver by the following formula: 


SoLuTIon 1 SOLUTION 2 
Water 800 cc. Water 800 cc. 
20 per cent Rochelle salt 20 per cent Rochelle salt 
solution 200 cc. solution 200 ce. 
40 per cent silver nitrate 30 per cent crystalline 


solution 50 ce. ferrous sulfate solution 107 ce. 


44. COLLOID CHEMISTRY 


Solution 2 was then added to Solution 1. The 
Rochelle salt here acts as a protector. 

In the photographic field the so-called Lippmann 
‘‘grainless” emulsion is made by the following formula: 


Sotution 1 SoLuTIoN 2 
Gelatin 75 grains Gelatin 75 grains 
Potassium bromide 3 grains Silver nitrate 45 grains 
Water 8 oz. Water 8 oz. 


The ingredients are mixed in the order given; and 
after allowing the gelatin to swell, the solutions are 
heated to 95° F. Then Solution 2 is added to Solu- 
tion 1. 

Lobry de Bruyn * found that, upon adding gelatin 
to both reacting solutions, precipitation may in many 
cases be inhibited, a colloidal dispersion resulting. 
Thus, under these conditions potassium bichromate 
and silver nitrate give a brick-red coloration but no ~ 
precipitate. In general, Lobry de Bruyn used 0.1 
to 0.05 N solutions, protecting them with 5 to 10 
per cent of gelatin. 

The value of this principle in producing colloidal 
precipitates is obvious, especially for colors, insecti- 
cides, pharmaceutical preparations, etc. It is also of 
the highest importance in biology and medicine, and 
may serve to explain many anomalies in physiology, 
pathology, and related branches, where things happen 
wm vivo that are not duplicated in vitro. Almost no 
reactions occur in the organism which are not con- 
trolled by the protective colloids everywhere present. 


* Rec. trav. chim., 19 (1900), 236; Ber., 35 (1902), 3079. 


GENERAL PROPERTIES OF COLLOIDS 45 


Autoprotection 


Many substances are naturally prone to assume the 
colloidal state, and in a large number of these it ap- 
pears that the molecular aggregates first reaching the 
colloidal degree of dispersion where surface forces 
predominate adsorb the less aggregated groups or 
isolated molecules, and thus the advance toward 
visible crystallization is delayed, or, for all practical 
purposes, entirely inhibited. Such a condition occurs 
in sulfur and also in pure iron, where one allotrope, 
y-iron, seems to be adsorbed by a-iron (ferrite). Am- 
monium salts, in particular, and some oleates exhibit 
the phenomenon. The globulitic stage is often a 
precursor to crystallization, but conditions may pre- 
vent emergence from this stage. Lactose acts this 
way. Several remarkable cases of autoprotection 
have been reported and these will be briefly con- 
sidered. 

W. B. Hardy * found that 5-dimethylaminoanilino- 
8, 4-diphenyleyclo-1, 2-dione, upon cooling from its 
solutions in organic solvents, gives gels that gradually 
become crystalline. Gortner and Hoffmannf re- 
port that dibenzoyl-l-cystine forms even in 0.2 per 
cent solution a rigid gel, which in the course of several 
weeks crystallizes almost completely. So powerful is 
its gelatinization that this dilute solution makes as 
_ strong a jelly as a 5 per cent solution of gelatin. 
‘Camphorylphenylthiosemicarbazide acts similarly, its 
solutions in organic solvents forming, on quick cooling, 
gels that gradually become crystalline.t 


* Proc. Roy. Soc. (London), 87A (1913), 29. 

tJ. Am. Chem. Soc., 43 (1921), 2199. 

t Forster and Jackson, J. Chem. Soc., 91 (1907), 188; E. Hatschek, 
Kolloid Z., 11 (1912), 158. 


46 COLLOID CHEMISTRY 


Cumulative Protection 


The general rule in colloidal protection is that any- 
thing which removes, coagulates, or destroys the pro- 
tector, or disperses it crystalloidally, will cause the 
protected dispersion to coagulate. Thus, on adding 
ferric chloride to olive oil emulsified with gum arabic, 
the gum is coagulated and loses its emulsostatic action, 
so that the oil separates out—the emulsion ‘ breaks.”’ 
On the other hand, anything that protects the pro- 
tector—that is, stabilizes it against coagulation, 
crystalloidal dispersion, or destruction—will tend to 
stabilize the protected sol. Thus, some ammonia in 
a gelatin-protected sol would tend to prevent the 
coagulation of the gelatin by formaldehyde, and an 
antiseptic would prevent its dispersion by bacteria or 
enzymes which, as the work of E: Zunz indicates, 
may convert protectors into coagulants. The rennin 
coagulation of milk seems to be of this nature, the 
enzyme converting the protective lactalbumin into 
a coagulant.* 

This introduces a new idea—cumulative protection. 
In many cases we must expect to find that the pro- 
tector of the protective colloid is itself a colloid, and 
there is no reason to doubt that cumulative protection 
may extend through a series of several colloids, or 
colloids and crystalloids, giving a structure which, 
like a house of cards, may collapse if any one of its 
essential supports is removed. 

This aspect of protection does not seem to have been 
recognized or investigated experimentally as such, and 
it offers a wide field for research. As instances of its 
practical operation may be mentioned glasses, alloys, 


* Alexander, 8th Intern. Cong. Appl. Chem., 1912. 


GENERAL PROPERTIES OF COLLOIDS 47 


and the gluten of wheat and rye flours. Many protein 
and similar complexes may be built up in this fashion. 


Dialysis 

Colloid solutions possess a small but definite diffusi- 
bility through colloidal septa (parchment paper, 
bladder) as was recognized by Graham, who found 
that “‘tannic acid passes through parchment paper 
about 200 times slower than sodium chlorid; gum 
arabic 400 times slower.’”’ * Graham/’s original form 
of dialyzer may be made from a wide-mouthed bottle 
whose bottom has been removed.t The mouth is 
closed by a piece of bladder or parchment paper 
tightly bound on, the solution to be dialyzed is poured 
in, and the bottle immersed about halfway in water 
eontained in a larger vessel. Most of the crystalloids 
diffuse through the membrane into the outer water, 
which should be frequently renewed, while most of the 
colloids remain in the original bottle, and may be thus 
obtained in a purified condition. Improved modern 
dialyzers consist of parchment or collodion sacs or 
thimbles, or even of whole bladders, which have the 
advantage of a larger dialyzing surface. 


Ultrafiltration 


H. Bechhold found that he could make filtering 
membranes of varying degrees of permeability by 
forming them from jellies of varying concentration. 
He used principally collodion dissolved in glacial 
acetic acid and afterward immersed in water, and 


* It is commonly but erroneously stated, even in text books, that 
colloids do not diffuse or dialyze. The extent to which dialysis occurs 
depends mainly on the fineness of the colloidal particles, the nature of 
the septum, and time. 

t A lamp chimney will answer very well. 


48 COLLOID CHEMISTRY 


gelatin jellies hardened in ice-cold formaldehyde. 
The jellies were formed and hardened on pieces of 
filter paper, which were supported from below by 
nickel wire cloth, and clamped between two flanges. 
The liquid to be subjected to ultrafiltration is intro- 
duced in the chamber thus formed and forced through 
the prepared septum by appropriate pressure, which 
may run up to 20 atmospheres or more and may 
be produced by a pump or by compressed gas (air, 
nitrogen or CO:). Table VII (below), prepared by 
Bechhold, shows various colloids arranged in order of 
the diminishing size of their particles in solution, and 


TABLE VII 
Suspensions. 
Prussian blue. 
Platinum sol (made by Bredig’s method). 
Ferric oxid hydrosol. 
Casein (in milk). 
Arsenic sulphid hydrosol. 
Colloidal gold hydrosol (Zsigmondy’s No. 4, particles about 40 uy). 
Colloidal bismuth oxid (Paal’s “Bismon”’), 
Colloidal silver (Paal’s “Lysargin’’). 
Colloidal silver (von Heyden’s “Collargol,’” particles about 20 up). 
Colloidal gold hydrosol (Zsigmondy’s No. 0, particles about 1-4 bm). 
Gelatin solution, 1 per cent. 
Hemoglobin solution, 1 per cent (molecular weight about 16,000). 
Serum albumin (molecular weight about 5000 to 15,000). 
Diphtheria toxin. 
Protalbumoses. 
Colloidal silicic acid. 
Lysalbinic acid. 
Deuteroalbumoses A. 
Deuteroalbumoses B (molecular weight about 2400). 
Deuteroalbumoses C. 
Litmus. 
Dextrin (molecular weight about 965). 
Crystalloids. 


GENERAL PROPERTIES OF COLLOIDS 49 


was obtained by using ultrafilters of varying degrees 
of porosity or permeability. 

By means of ultrafiltration through ultrafilters of 
appropriate permeability, not only may colloids be 
separated from crystalloids, but colloids having par- 
ticles of different sizes may be separated from each 
other. 

Diffusion 

Diffusion through a septum is, of course, involved 
in dialysis. If, however, diffusion occurs into a jelly, 
many interesting phenomena may develop, especially 
if the jelly adsorbs any of the diffusing substances or 
contains substances which can react with them. 

Owing to the enormous surface they present, col- 
loidal gels exhibit a powerful adsorptive action. In 
fact, even when percolated through such a relatively 
coarse-grained septum as sand, most solutions issue 
with a materially reduced content of solute, and 
benzopurpurin solutions may be thus decolorized. 
Further, if a solute hydrolyzes into ions having dif- 
ferent degrees of adsorbability or different rates of 
diffusibility, they may be actually separated by 
diffusion through a colloidal gel. 

This phenomenon is nicely exhibited by what may 
be termed a ‘“‘patriotic test tube,’’ prepared by filling 
a tube about two-thirds full with a slightly alkaline 
solution of agar containing a little potassium fer- 
rocyanid and enough phenolphthalein to turn it pink. 
After the agar has set to a firm gel, a solution of ferric 
chlorid is carefully poured on top, and almost instantly 
the separation becomes evident. The iron forms 
with the ferrocyanid a slowly advancing band of blue, 
before which the.more rapidly diffusing hydrochloric 


ty 2 ; Diffusion.) 


50 COLLOID CHEMISTRY 


acid spreads a white band as it discharges the pink of 
the indicator. After the lapse of a few days the tube 
is about equally banded in red, white, and blue. 

Even then the tubes do not cease to be of interest, 
for if they are allowed to stand several weeks the pink 
color is all discharged and there develop peculiar bands 
or striations of blue, apparently due to the fact that 
the iron ferrocyanid temporarily blocks the diffusion 
passages, which are gradually opened again after a 
layer of the blue salt has diffused on from the lower 
surface. (Liesegang’s rings.*) 

Not only may ions be thus separated, but if two 
solutes in the same solvent possess different rates of 
diffusion or different degrees of adsorbability, they 
also may be separated from each other by diffusion 
through a colloidal gel or septum. (Differential 


Electric Charge and Migration 


| /the particles of practically all colloidal solutions 


possess an electric charge, and under the influence of 
an electric current (difference of potential) move 
toward the electrode having the opposite charge. 
(Electrophoresis.) In general, when two substances 
are brought into contact, the one having the higher 
dielectric constant becomes positively charged, whereas 
the one with the lower dielectric constant becomes 
negatively charged (Cohen’s Law). Since water has a 
high dielectric constant (80), most substances sus- 
pended in pure water become negatively charged and 
wander to the anode. On the other hand, if suspended 
in oil of turpentine, which has a low dielectric constant 

* Liesegang’s rings, named after Raphael Ed. Liesegang, are of 


importance in mineralogy (agates), geology (ore deposits), and biology 
(rhythmic banded structures). 


GENERAL PROPERTIES OF COLLOIDS 51 


(2.23), they become positively charged and wander to 
the cathode. 

If, however, electrolytes are present, Cohen’s law is 
superseded by other controlling factors, such as the 
adsorption of ions, which may give their charge to the 
suspended particles. In fact Hardy found that in pure 
water albumin was amphoteric; in the presence of a 
trace of alkali it acquired a negative charge and 
migrated to the anode; but a trace of acid gave it a 
positive charge and it then migrated to the cathode. 
The following table shows the usual charge and migra- 
tion tendency of a number of aqueous colloidal solu- 
tions. 


Charged + Charged — 
Migrate to Cathode (— Pole) Migrate to Anode (+ Pole) 
1. Hydrates of Fe, Cu, Cd, Al, Zr, 1. Sulphids of As, Sb, Cu, Pb, Cd. 
Ce, Th. Halides of Ag. 

2. Titanic acid. 2. Stannic acid, silicic acid. 

3. Colloidal Bi, Pb, Fe and Cu 3. Colloidal Pt, Au, Ag, and Hg, 

(Bredig’s method). I, 8, Se. 

4, Albumen, hemoglobin, agar. 4. Gum arabic, soluble starch, 
gamboge, mastic, oil emul- 
sion. 

5. Basic Dyes: Methyl violet, 5. Acid Dyes: Eosin, fuchsin, 

Bismarck brown, methylene anilin blue, indigo, soluble 
blue, Hofmann violet. Prussian blue. 


The electrically charged particles tend to, and 
usually do, surround themselves with ions of the 
opposite charge, forming thus an electric double layer, 
as it is called. Or polar molecules may be oriented 
at the interface between the particle and the dispersion 
medium. | 


52 COLLOID CHEMISTRY 


Pectization and Peptization 

Briefly stated, pectization means the coagulation of 
a colloidal sol, and peptization its redispersion. If a 
small quantity of an electrolyte is added to a pure 
ruby red colloidal gold solution, the latter changes 
to a blue or violet color, and deposits its gold as a 
fine blackish coagulum or precipitate. By watching 
in the ultramicroscope the coagulation of very dilute 
milk by dilute acid, the individual particles of the 
colloidal casein may be seen to gather gradually 
together into groups, whose motion becomes pro- 
gressively less as their size increases, until they are 
no longer able to stay afloat, and finally coagulate in 
large grape-like clusters. Hardy believes that the 
particles of colloids adsorb the oppositely charged 
ions of electrolytes present; at the isoelectric point 
(that 1s, when there is no excess either of positive or 
negative charges on the particles) coagulation occurs. 
If, however, an excess of electrolyte be added all at 
once, the isoelectric point may be passed before coagu- 
lation occurs, and the particles acquire a charge 
opposite to the one they had originally. Under such 
conditions, no coagulation may result. 

Burton epitomizes the difference in action of various 
electrolytes as follows: ‘‘Two remarkable results are 
evident on comparing the coagulative powers of 
various electrolytes on colloids of different kinds; first, 
the coagulation depends entirely on the ion bearing a 
charge of sign opposite to that of the colloidal particle; 
and, second, with solutions of salts, trivalent ions 
have, in general, immensely greater coagulative power 
than divalent ions, and the latter, in turn, much 
greater than univalent. Acids and alkalis in particular 
cases act more strongly than the corresponding salts.” 


GENERAL PROPERTIES OF COLLOIDS 53 


High-tension electric discharges may also effect the 
coagulation or precipitation of a finely subdivided or 
dispersed phase; which fact was utilized by Sir 
Oliver Lodge in dispelling fogs, and by Cottrell in 
coagulating smelter and similar fumes. 

Peptization.—So strong is the analogy between 
digestion and colloidal disintegration that Thomas 
Graham, the father of colloid chemistry, coined the 
word peptization to express the liquefaction of a gel. 
He first speaks of the coagulation or pectization of col- 
loids. ‘‘The pectization of liquid silicic acid,” he 
states, ‘‘and many other liquid colloids is effected by 
contact with minute quantities of salts in a way which 
is not understood. On the other hand, the gelatinous 
acid may be again liquefied, and have its energy 
restored by contact with very moderate amounts of 
alkali. The latter change is gradual, 1 part of caustic 
soda, dissolved in 10,000 water, liquefying 200 parts 
of silicic acid (estimated dry) in 60 minutes at 100. 
degrees. Gelatinous stannic acid also is easily 
liquefied by a small proportion of alkali, even at the 
ordinary temperature. The alkali, too, after lique- 
fying the gelatinous colloid, may be separated again 
from it by diffusion into water upon a dialyzer. The 
solution of these colloids in such circumstances may be 
looked upon as analogous to the solution of insoluble 
organic colloids witnessed in animal digestion, with the 
difference that the solvent fluid here is not acid but 
alkaline. Liquid silicic acid may be represented as 
the ‘peptone’ of gelatinous silicic acid; and the 
liquefaction of the latter by a trace of alkali may be 
spoken of as the peptization of the jelly. The pure 
jellies of alumina, peroxide of iron and titanic acid, 

5 


54 COLLOID CHEMISTRY 


prepared by dialysis, are assimilated more closely to 
albumen, being peptized by minute quantities of 
hydrochloric acid.” 

Peptization is in reality deflocculation, a dispersion 
of groups into separate particles which once more 
acquire active motion and remain afloat or in solution. 
The detergent action of soap and dilute alkalis is due 
to the fact that they deflocculate adhering particles of 
“dirt.” | 

Viscosity 

Viscosity depends largely on the relation between 
kinetic motion and free or specific surface. It may 
be measured by time of flow from an orifice under a 
fixed head or pressure (Ostwald or Engler types), or 
by measuring the force needed to shear layers of liquid 
past each other (Couette or MacMichael types). 
Crystalloid solutions exhibit low viscosity; the rapid 
motion of their particles aids flow. The relatively 
coarse suspensions are likewise not viscous, because 
of deficiency of active surface. In the colloidal zone 
we find maximum viscosity, which falls off on either 
side (zone of maximum colloidality, see p. 16 et seq.). 

Slowly moving particles exhibit inertia to the com- 
mencement of flow, and this accounts for what is 
known as ‘‘plasticity’’ or ‘‘yield value’”’ of viscous 
materials. Theoretically everything should have some 
yield value, though in many cases it is not measurable, 
especially over limited time.* 


* See J. Alexander, ‘‘Glue and Gelatin,” p. 98 et seq. 

Thomas Graham said (Proc. Roy. Soc. Lond., 1864): The ultimate 
pectization of silicic acid is preceded by a gradual thickening in the 
liquid itself. The flow of liquid colloids through a capillary tube is 
always slow compared with the flow of crystalloid solutions, so that a 
liquid transpiration-tube may be employed as a colloidoscope. With a 
colloidal liquid alterable in viscosity, such as silicic acid, the increased 
resistance to passage through the colloidoscope is obvious from day 
to day. Just before gelatinizing, silicic acid flows like an oil. 


CHAPTER 7 


PRACTICAL APPLICATIONS OF COLLOID-CHEMICAL 
PRINCIPLES 

The practical applications of colloid chemistry are 
so manifold and widespread that they touch every 
branch of science and technology. Whole books may 
be and have been written on many of the most restricted 
fields, while the scientific literature teems with 
monographs and articles, directly on, or applicable to, 
colloid-chemical subjects. In what follows, it will be 
possible therefore to give not an exhaustive, but only 
a most general survey, intended primarily to show the 
ubiquity of colloid phenomena; and many important 
topics must be dismissed with a most rudimentary 
discussion, altogether incommensurate with their 
importance. 

Practically all the substances we meet with in our 
everyday life are colloids, and we are mainly colloids 
ourselves. The foods we eat, the clothes and shoes 
we wear, the wooden furniture we use, the houses we 
live in with their windows, carpets and floors, the 
metals and rubber of our automobiles as well as their 
leather or leather substitutes, the very books and 
newspapers we read, and the paper and twine that 
our purchases come wrapped in—all are largely if not 
entirely composed of colloids or matter in the colloidal 
condition. Consequently, problems in colloidal chem- 
istry appear in every branch of science, industry and 
technology. 

It is intended here to give in outline a sketch of a 
number of these applications, stressing more particu- 

55 


56 COLLOID CHEMISTRY 


larly those that have biological technological bearings, 
but including others that show the operation of the 
colloid-chemical principles involved. In order to 
bring some coherence into this extensive survey, the 
topics are gathered into more or less related groups 


as follows: 


Astronomy 
Meteorology 
Smokes and Dusts 
Perfumes 


Agriculture 
Clay 


Dyeing 
Shower-proofing of 
Fabrics 


Soaps 
Lubrication 
Coal 

Colloidal Fuel 


Filtration 
Sewage Disposal 
Photography 


I 


Geology 
Mineralogy 
Gems 


II 


Ceramics and Refractories 
Flotation 


III 


Nitrocellulose and _ Its 
Products ? 

Paints, Pigments and Var- 
nishes 

Emulsions 


IV 


Petroleum 
Asphalt 
Fire Foam 
Insecticides 


Vv 


Brewing 
Tanning 
Paper 
Rubber 


PRACTICAL APPLICATIONS 57 


VI 
Foods Ice Cream 
Baking Confectionery 
Milk Gelatin and Glue 
VII 
Glasses Boiler Scale 
Metals and Alloys Cement, Mortar and Plas- 
Electro-deposition of ter 
Metals 
VIII 
Chemical Analysis Antiseptics and Bacteri- 
Pharmacy and Thera- ology 
peutics Biology and Medicine 
Astronomy 


As matter in colloidal state is so common on our 
relatively minute earth, it is but natural to expect to 
find many instances of colloidal dispersion in the 
immensity of the Universe. 

Cosmic dust is widely distributed throughout space, 
and as it is gathered up by the superior attraction of 
the larger heavenly masses (suns, planets, etc.), which 
in any system grow at the expense of the smaller 
masses, fresh quantities are continually produced by 
the collisions of bodies in space, as well as the disinte- 
gration of meteorites, comets, asteroids, etc. 

According to Isabel M. Lewis of the U. S. Naval 
Observatory, a swarm of meteors or ‘‘shooting stars” 
(which are quite different from the large meteorites 
or fire balls) consists of minute dust-like particles 
which for the most part do not weigh as much as a 


58 COLLOID CHEMISTRY 


single grain. If we could view them outside our at- 
mosphere, they would appear as a cloud of dust reflect- 
ing the sunlight, their luminescence being caused by 
the friction of their rapid passage through our atmos- 
phere. Particles of colloidal size, which probably 
accompany them through space, tend to be retarded 
by the viscosity of our atmosphere. 

The zodiacal light also seems to be consequent on a 
cloud of colloidal matter generally believed to sur- 
round the sun. Lars Vegard recently advanced the 
view that the upper atmosphere contains minute 
crystals of frozen nitrogen, to which he attributes the 
blue color of the sky, as well as the aurora borealis and 
the zodiacal light (see Philosophical Transactions, 
Oct. 1923). 

When enormous nebulous or fluid masses moving 
through space approach each other, they cause huge 
‘tides’? in each other, and as the bodies pass, these 
tides are converted into swirls (spiral nebulae) which 
may later condense into planetary units about the 
residual central sun (solar system). This view ac- 
counts for the fact that our planets all revolve in one 
direction, and it furthermore allows for the persistence 
of colloidally dispersed residues, as well as ‘‘captured”’ 
matter. The retrograde moon of Jupiter is probably 
a ‘‘captured”’ satellite. | 

Heavenly bodies must occasionally meet in collision, 
and the terrific impact must add to the nebulous or 
cometary masses of colloidal matter which are distrib- 
uted throughout space.* While most of these bodies 


* In contradistinction to phenomena of such magnitude Prof. R. A. 
Millikan (Nobel prize winner) weighed the electron by catching these 
minute individual particles of negative electricity on colloidal oil drop- 
lets which he viewed in an ultramicroscope. . 


PRACTICAL APPLICATIONS 59 


are self-luminous, others are lighted up by neighboring 
suns. Light itself is invisible, but a beam of sunlight 
shot into a darkened room becomes evident by illu- 
minating the innumerable motes in our atmosphere, 
most of which are colloidal. Some of these atmos- 
pheric nuclei, as C. T. R. Wilson has shown, are due 
to the deposit of moisture, etc., on ions or ionized 
particles, but others are without charge. 

A few years ago when, as astronomers had foretold, 
the earth was about to pass through the tail of a 
comet, our yellow journals warned us to prepare for 
the end—but no one, not even the astronomers, 
noticed the slightest effect. The tail, whose length 
ran into millions of miles, was only a vast cloud of 
colloidally dispersed extremely tenuous matter, and 
its luminosity was simply a Faraday-Tyndall effect 
on a gigantic scale, but analogous to the beam which 
a searchlight or automobile headlight shoots through 
the atmosphere, especially if it is slightly hazy. The 
spectroscope indicates that the luminosity of the tail 
is due to reflected sunlight. 

The tails of comets seem to consist almast entirely, 
and the nuclei and coma largely, of colloidally dis- 
persed matter. The great comet of 1882 which made 
a transit of the sun was invisible against the solar dise 
(a position corresponding to attempted observation of 
colloidal particles in the ordinary microscope against 
a luminous background), but became visible again 
after passing beyond the sun’s disc (a position cor- 
responding to successful observation of the same col- 
loidal particles in the ultramicroscope against a dark 
background, the eye of the observer being protected 
from the source of illumination). 


60 COLLOID CHEMISTRY 


The streaming of the cometary tails away from the 
sun may be due to the ionization of the constituent 
colloidal particles, and their consequent electrical 
repulsion. With colloidal particles the Brownian or 
kinetic motion is a factor to be reckoned with. J. 
Clerk Maxwell pointed out (1870) that the intensity 
of the action of the sun’s rays on a particle depends 
upon its surface, which varies as the square of its 
diameter, whereas the gravitation of the same particle 
to the sun depends upon its mass, which varies as 
the cube of its diameter. Theoretically in the case 
of a particle whose density equals that of water, 
the repulsion balances gravitation when the diameter 
reaches 0.0015 mm. (=1.5y). As the diameter 
diminishes, the repulsive force gains the ascendancy, 
soon reaching a maximum and again diminishing, until 
when the particle has a diameter of only 0.00007 mm. 
(= 70 up) the two forces again balance each other.* 

These figures, which refer to a substance having the 
density of water, are approximately of colloidal dimen- 
sions; but in the case of denser bodies the subdivision 
would be.even more profound. It is therefore not 
surprising that, when the earth recently passed through 
the tail of a comet, no disturbance of any kind was 
noticed. ‘The comet’s tail is a vast celestial camou- 
flage—its luminosity a macroscopic Faraday-Tyndall 
effect. From what is stated above we should imagine 
that in some cases the tail of a comet may point away 
from the sun, providing that its particles are of the 
proper size, and in fact Baade’s comet, recently visible, 
showed this phenomenon. Comets have been seen 


* See Simon Newcomb’s article on “Comet,’’ Encyclopedia Britan- 
nica, 11th edition. Also Svante Arrhenius, ‘Worlds in the Making,” 
Harper & Bros. 


PRACTICAL APPLICATIONS 61 


with tails pointing both towards the sun and away 
from it. 

The nebule, too, apparently consist of finely dis- 
persed matter, rendered luminous by neighboring suns; 
although with them, as with the comets, part of the 
light may result from self-luminescence (incandescent 
gas, etc.). 

Meteorology 

What we commonly call ‘‘weather conditions”’ are 
largely dependent upon the degree of dispersion of 
water in the atmosphere, and this dispersion is mainly 
effected and maintained by solar heat and electrical 
energy. When air carrying water vapor is chilled 
by rising to a higher level (which means reduced 
pressure and expansion), meeting a colder mass of 
air, or even by the alternation of night and day, the 
moisture it contains assumes the colloidal state as 
cloud, fog or mist; and as the coagulation of the 
_dispersed water proceeds, these in turn may condense 
still further into dew, rain, snow or hail, depending 
upon conditions. 

When steam under pressure blows off into the atmos- 
phere, it appears clear some distance from the point 
of issue and does not become visible until it has con- 
densed into colloidal dimensions. ‘Then as its particles 
condense still further to drops (or else evaporate or 
are dispersed), it becomes invisible once more. White 
cloud particles are already larger than strictly colloidal 
size. 

As the water particles in a cloud aggregate, the total 
specific or ‘‘free’’ surface diminishes, but must con- 
tinue to hold the total electric charge because the 
charge distributes itself superficially. This means 


62 COLLOID CHEMISTRY 


that the charges on the outside of the individual 
droplets increase enormously until the lightning dis- 
charge occurs.* We have all noticed how a nearby 
lightning flash is promptly followed by an increased 
fall of raindrops whose condensation gave it birth. 
The condensation of the moisture particles usually 
shows itself by the cloud assuming a dark gray or 
bluish-black shade. 

Many water droplets are formed about ions or 
electrons and carry an electric charge. As the 
droplets aggregate the charge accumulates on the 
outer surface, so that in a large cloud, with an enor- 
mous number of droplets, very high potentials may 
result. Indeed if the aggregation of the water droplets 
is not sufficient to obscure the sky, and the charges 
are very numerous, we may have a “bolt out of the 
blue’’—not an uncommon phenomenon. 

We do not know to what extent electrical conditions 
on the earth affect the dispersion of substances in its 
atmosphere; but since half of the earth is always 
heated by the sun while the other half is cooler, 
thermoelectric currents are continually circulating 
about the earth. Variations in solar radiation, due 
to sun-spots and the like, cause violent electric and 
magnetic storms which are intimately connected with 
the aurora, and other atmospheric phenomena (ioniza- 
tion, electrical charge of dispersed particles). 

Colloidal ice often occurs at high levels and may be 
the cause of halos, perihelions and periselenes (‘‘sun- 
‘dogs’ and ‘‘moon-dogs’’), etc. In many elevated 
regions (Idaho) there occur frost fogs called by the 


* It is possible that in the gymnotus and other “electric fish,” an 
aggregation of colloids in their special glands, due to nervous influences, 
may underlie the high potential and shock. 


PRACTICAL APPLICATIONS 63 


Blackfoot Indians ‘‘pogonip”’ or ‘‘ white death,’’ where 
finely divided ice, like hoar-frost on a gigantic scale, 
deposits on trees in huge masses, and is a contributing 
cause of pneumonia. 

Since changes in barometric pressure must affect the 
CO, tension of the blood and tissues, and since this 
means a variation in the turgidity of the body col- 
loids which incidently affects the circulation (see 
p. 184), we have here one basis for the popular im- 
pression that the ‘‘weather”’ influences our feelings. 
Other factors are of course at work (e.g., evaporation 
of water from the skin, etc.), but physicians know 
that with persons suffering from neurotic and rheu- 
matic affections, the effects of weather conditions are 
noticeable. Change of altitude may have a.similar 
effect. 

The blue color of the sky is caused by diffraction of 
the sunlight by the earth’s atmosphere. Were it not 
_ for this gigantic Faraday-Tyndall effect the sun would 
appear to us as a fiery ball set in a black star-sprinkled 
sky. The varying shades of bodies of water (‘‘blue 
Mediterranean,” ‘‘green’’ ocean, black, yellow, white, 
red rivers, etc.) are largely due to the diffraction 
caused by finely dispersed substances. When we look 
through a great length of the atmosphere, distant 
objects such as mountains seem to have a blue tone. 

Tis distance lends enchantment to the view, 
And robes the mountain in its azure hue. 

So the poet Thomas Campbell beautifully puts it 
in his ‘‘Pleasures of Hope.’”’ Though atmospheric 
gases exercise some effect, without colloidal dust 
we would have less romantic twilight, and darkness 
would fall rather sharply as the sun sank below the 
horizon. 


64 COLLOID CHEMISTRY 


The blue-whiteness of snow, which sometimes even 
assumes a positive blue tint, is due to the reflection of | 
light from its interior surfaces, the color being a struc- 
tural one. [See J. Alexander, Science, 41, 465 (1920).] 
As Bancroft has pointed out, the blue color of many 
birds’ feathers is due to their colloidally fine structure. 
A certain green tree toad owes its color to a structural 
blue, which on passing through a layer of yellow oil 
droplets in the skin appears green. If preserved in 
alcohol, which dissolves out the oil, the toad turns 
blue. 

The tremendous explosive eruption of the voleano 
Krakatoa in 1883 projected colloidal dust and ashes so 
high that they gradually spread entirely around the 
earth. Besides causing glorious ‘‘golden sunsets,”’ 
such fine dust is by many believed to exert a 
noticeable influence upon weather conditions. While 
many particles in such dust greatly exceed colloidal 
dimensions, it must be remembered that the effects of 
fine subdivision do not end abruptly with the usual 
colloidal limits of 5 to 100 millimicrons. Test plates 
taken in an airplane, show that pollen grains are 
numerous at an altitude of 15,000 feet (Hay fever). 

One more curious function of colloidal atmospheric 
dust must be mentioned. As Kendall has shown, the 
proper functioning of the thyroid gland depends upon 
the presence of a small quantity of thyroxin, an 
iodine-containing organic compound. This means 
that iodine must be found in traces in the soil, from 
which all food directly or indirectly comes. The origi- 
nal source of iodine is the sea, and although some of it 
exists in geological deposits of sea salt, a continual 
source of supply to the soil is to be found in the col- 


PRACTICAL APPLICATIONS | 65 


loidal sea-salt dust, which forms from the ocean’s spray 
and is carried thousands of miles inland before being 
washed down by therain. The incidence of goiter and 
cretinism (largely due to iodine insufficiency) is most 
marked in interior mountainous (Switzerland, Idaho, 
Wyoming) regions where the least amount of sea-salt 
dust is apt to be carried. Thus it would take a person 
1,000 years to drink enough of the water of Lake 
Superior to give the iodine needed for his thyroid 


gland. 
Smokes, Fogs and Dusts 


(Aerosols) 


Fogs, mists, and clouds, whether of water or dust, 
exercise a potent influence on terrestrial climate, flora 
and fauna, for they govern to a considerable degree 
the amount of solar radiation reaching the earth’s 
surface. According to one theory, glacial periods 
were, in part at least, due to such influences. 

_ By interfering with the normal passage of solar 
radiations, as well as by furnishing nuclei for the de- 
posit of moisture to form fog, coal smoke, if present 
on a large enough scale (as in some parts of England), 
may exert an effect upon weather conditions. Carl 
Barus [Smithsonian Contribution 1309 on ‘‘ Atmos- 
pheric Nucleation,’’? Washington, 1905] kept a con- 
tinuous record of atmospheric nucleation for several 
years and found that the number of nuclei varied from 
about 2,000 to 100,000 per cubic centimeter. Such 
nuclei may be produced by violently agitating liquids, 
by heated bodies (red-hot metal or glass, a clear non- 
luminous bunsen flame), by chemical activity (smoke- 
less oxidation of phosphorus at about 13 deg.), by 
evaporation (e.g., of sulphuric acid or naphthalene), by 


66 COLLOID CHEMISTRY 


high potential (charged metal), and by certain kinds 
of radiation (ultra-violet light, X-rays, radioactivity). 
Barus says that the nuclei are at the outset simply dust 
particles small enough to float in the air, but larger 
than the order of molecular size, upon which water 
condenses. Most of them carry an electric charge. 
Atmospheric nucleation varied greatly from day to day 
and during the day, but was on the average greatest at 
the winter solstice (December 21) and least at the 
summer solstice (June 21). 

In flames, the burning matter is largely in colloidal 
state, and when a combustible dust suspended in air 
approaches colloidal dimensions, its free surface be- 
comes so great that it may burn with extreme rapidity 
if started by a static or frictional spark or the like 
(explosive aerosols or dusts). 

Sodom and Gomorrah were probably destroyed by 
a colloidal cloud of petroleum ignited by local fires or 
lightning. Analogous phenomena are known in the 
Caucasus Mountains to-day. 

The electrical precipitation of colloidal and finely 
dispersed substances from air, smoke, furnace gases, 
etc., by the Cottrell process is too well known to be 
described in detail here. Unexpected sources of profit 
are being found in the fumes compulsorily collected 
from cement kilns (potash) and from smelters (arsenic, 
selenium, etc.). Many dusts and mists (including 
sulphuric acid, which is a very good nucleator) that 
had defied ordinary bag-house methods are readily 
separated out by the high tension current. Sir Oliver 
Lodge even suggested that ships might thus clear their 
path of fog for some distance ahead. 

Recently W. D. Bancroft and L. F. Warren demon- 


PRACTICAL APPLICATIONS 67 


strated that electrified sand, scattered by an airplane, 
could cause the coagulation of fog and clouds, thus 
producing even a slight localized ‘‘ rain.’ 

One case where smoke is intentionally produced is 
in the use of ‘‘smudge pots,’’ which are burned in 
orchards in the spring or early summer when indica- 
tions are that the tender blossoms or fruit are about 
to be injured by frost. This remedy is, of course, 
ineffective in very cold weather, but often serves to 
prevent what agriculturists term a light frost, which 
has for its precursors a still, clear atmosphere and a 
temperature approximating the freezing point. Frosts 
of this kind do not occur on cloudy or windy nights. 

In the absence of smoke, the freezing process seems to 
proceed as follows: With falling temperature frost may 
deposit directly, or a dew-like deposit of water forms 
on leaves, buds, fruit and similar surfaces, and is con- 
gealed into ice crystals or ‘‘frost’”’ when the slight heat 
liberated by the condensation radiates into the sur- 
rounding cold atmosphere; the freezing is facilitated 
by the evaporation of part of the deposited water. 
While the heat generated by the burning smudge oil 
is a large factor, the smoke liberated furnishes through- 
out the entire lower atmosphere innumerable nuclei 
upon which moisture deposits, with the liberation of 
some heat and the formation of a protective fog 
blanket that tends to check terrestrial radiation. 
The combination of all these factors is usually suffi- 
cient to ward off the threatened calamity. 

These views are strengthened by the observations of 
Lieutenant B. H. Wyatt (Science, Sept. 21, 1923, p. 
223) who, during the recent total eclipse of the sun, 
took observations while flying in an airplane in a 


68 COLLOID CHEMISTRY 


comparatively clear layer at an altitude of 13,000 ft. 
“During totality the recording thermometer at this 
altitude showed a rise of three and one half degrees 
Fahrenheit instead of the anticipated drop period. 
The humidity fell from sixty-three to fifty-two per 
cent.”? This seems to indicate a liberation of heat 
upon the condensation of moisture, providing he did 
not enter a different stratum of air. 

The recent war resulted in an enormous develop- 
ment of the use of artificial smokes and fogs in mili- 
tary and naval offensive and defensive operations. 
While in the old days attacks or retreats had often 
been made under cover of natural fog or mist, now 
soldiers crept to the attack under the protection of a 
smoke barrage, and on sea destroyers laid down a 
smoke screen, or steamers sought, like the cuttlefish, 
to escape submarines in colloidal clouds of their own 
making. In the air natural clouds were lurking places 
for airplanes and dirigibles, and many air craft, es- 
pecially Zeppelins, produced colloidal clouds in which 
they were lost to view. 


Perfumes 


Without attempting to go into the question of what 
constitutes the sense of smell from a nervous or psy- 
chological standpoint, it may be said that the sensa- 
tion of smell or odor is aroused only by the transmission 
of material particles which must be very small, since 
they pass readily through paper, textiles and the small- 
est crevices, and are held back only by airtight sur- 
faces like glass or metal. Some gases have distinct 
odors, but from the fact that Barus [‘‘The Structure 
of the Nucleus,’ Smithsonian Contribution 1373, 


PRACTICAL APPLICATIONS 69 


Washington, 1903] was able to demonstrate the pres- 
ence of colloidal nuclei in gasoline, benzol and carbon 
disulphide vapors, it is obvious that the presence of 
such particles is much more common than has been 
believed. It is a question whether these nuclei arise 
from molecular association or from the condensation 
of vapor on smaller atmospheric nuclei. The persist- 
ency of naphthalene nuclei and their tendency to reach 
a fixed diameter led Barus to remark that ‘‘it is prob- 
able that the vapor pressure of naphthalene, however 
small, is the cause of this long maintenance of the 
supply of nuclei, and one may suspect that other 
bodies with strong odors and which show a tendency 
to slow sublimation will be found in like degree nuclei- 
producing.”’ 

Ambergris evidently owes its body-producing effect 
in perfumes to the formation by it of nuclei which have 
the power of strongly condensing the vapors of the 
ethereal oils, etc., which constitute the odoriferous 
ingredients of perfumes. Thus Barus found that 
paraffin nuclei, made by shaking a benzol solution of 
paraffin, disappeared with relative rapidity. This 
field offers an interesting vista of research, which may 
lead to a cheaper or superior substitute for ambergris, 
among other things. 

In modern society, smell is a sense that is largely 
tabu, although throughout the whole animal kingdom 
it is of prime importance and utility. Wild animals 
are guided largely by it, and insects seem to be also. 
While some odors are due to molecularly dispersed 
particles, in many cases we have to deal with colloidal 
particles; and it will be of interest to investigate the 
permeability of septa to smells. The odors of many 

6 


70 COLLOID CHEMISTRY 


substances are due to impurities. Thus absolutely 
pure sulfuretted hydrogen, carbon disulfide and phenol 
are without marked odor, as are also pure indol and 
skatol. 
Geology 

Coming back to solid earth, let us consider the 
ordinary properties and behavior of the earth’s solid 
constituents. ‘These depend more upon their state of 
physical dispersion than upon their chemical constitu- 
tion, as may be seen by considering Atterberg’s classi- 
fication of minerals according to the size of their 
fragments: 


Diameter 
Boulders shou eet eas eee 2. amos te20 cm 
Pebbles.)s 03 23s are eae eee 20 cm. to 2 cm 
Gravel 2: @0 2 See eee eee 2. (em toe mm 
Sand (0 Fo anc 08 2s rene cern ee een 2 mm.to 0.2 mm. 
Barth: iit SN eae ope eee 0.2 mm.to 0.02 mm. 
LOaIN Se he cos ante ene eee 0.02 mm. to 0.002 mm. 
Clayun a Sane ean nee smaller than 0.002 mm 


The finer particles are more readily blown about by 
the wind and carried and deposited into sedimentary 
beds by water; they are more soluble and are more 
easily leached and decomposed by ‘‘weathering.’’ 
In mass the finer particles are less permeable to water, 
but possess a much higher capillarity and water- 
holding capacity, besides a powerful fixing or de- 
composing action on diffusing substances. Thus if — 
dilute potassium sulphate solution is percolated 
through a column of fine sand, free sulphuric acid is 
found in the percolate. 

Fine deflocculated clay is carried thousands of miles 
by rivers, and when finally coagulated by the salts of 
the ocean forms new land. Geological evidence shows 


PRACTICAL APPLICATIONS ral 


that this process has added about a thousand miles to 
the length of the Mississippi River, and owing to the 
enormous areas under cultivation (which permit more 
soil to be carried off) it is going on perhaps faster than 
ever. Its results may be seen at the mouth or delta 
of every large river—the Nile, Ganges, Hwang-Ho, 
Amazon, Euphrates. The great weight thus accumu- 
lated at certain weak places in the earth’s crust may 
cause slip along lines of geological fault. The recent 
(1923) destructive earthquake in Japan is believed to 
have had such an origin. 

Fine particles generally have been deposited in huge 
thick beds by glacial and other prehistoric waters, 
which have mostly graded them according to size. 
Such deposits, when cemented by pressure, dehydra- 
tion or igneous or metamorphic action, became slate, 
shale, sandstone, etc. When they enclosed pebbles, 
the result was ‘‘ pudding stone,”’ and these frequently 
_served as a matrix for fossils and as a means to pre- 
serve fossil footprints, ripple marks, and even rain 
splashes. 

Eozoon, long thought to represent the fossilized 
remains of primeval plant life, is probably nothing 
but an instance of rhythmic precipitation (Liese- 
gane’s rings), which often occur when precipitates 
are formed by diffusion into colloidal gels. 


Mineralogy 


Agates, onyx and dendrites are types of minerals 
formed by rhythmic precipitation (Liesegang’s rings), 
and ore bodies may be deposited by similar segregating 
action. Some minerals are themselves colloidal gels 
(e.g., opal, flint, bauxite) or result from other minerals 


72 COLLOID CHEMISTRY 


by weathering with subsequent gel formation (e.g., 
kaolin from kaolinite, serpentine from diabase). 
Lehner has recently shown that mere trituration is 
sufficient to bring silica into colloidal solution, and 
it seems that many quartz minerals have been de- 
posited, in part at least, from colloidal solution. The 
same applies to stalactites and stalagmites, for with 
a fluctuating carbon dioxide content, especially in 
the presence of protectors like humus, some colloidal 
calcium carbonate will be formed. 


Gems 


Many gem stones owe their beautiful colors to im- 
purities colloidally dispersed in them—the emerald to 
chromium, the ruby to iron, the amethyst probably to 
manganese, the topaz to iron, etc. The varying 
shades of diamonds seem to be due to similar causes. 
Opals are gels with interior reflecting planes. The 
shade and even the color of many gems can be changed 
by subjecting them to the action of X-rays or radium, 
which apparently act by bringing the dispersed par- 
ticles into a different state of aggregation, usually 
coarser. 

Pearl scales, when examined in the dark field, show 
the inhibited crystallization characteristic of crystals 
formed in the presence of a protective colloid, which 
here is the slime or mucin of the mollusc. There is no 
reason to doubt that colloid research will lead to the 
synthetic production of mother-of-pearl, and perhaps 
even of substitutes for the real pear! itself. 

Since the writing of the above there has appeared 
in ‘‘Chemie et Industrie,” Vol. 8, p. 782 (1922), a paper 
by Clément and Riviére, describing the production of 
artificial mother-of-pearl. 


CHAPTER 8 
PRACTICAL APPLICATIONS (Continued) 
AGRICULTURE 


Although from time immemorial farmers have 
classified soils on the basis of their physical and 
physiological character as “‘light”’ or ‘‘heavy,” ‘‘rich”’ 
or “poor,” ‘‘productive” or ‘‘unproductive,”’ etc., 
it is only within comparatively recent years that 
chemists have begun to realize the full importance of 
the réle played by the colloids, especially the organic 
colloids of the soil. 

The constituents of the soil fall into three classes: 

1. Mineral fragments, consisting mainly of silica 
and silicates, resulting from the disintegration of rocks 
by weather, water, heat, freezing, etc. 

2. Organic material, including bacteria, and that 
-mixture of plant and animal debris known as humus. 

3. Soil solution, which is water with its various 
solutes. ; 

The fact that colloids occur in all three of these 
classes should serve to reconcile the older hypothesis 
that the colloidal behavior of clay is due to fine par- 
ticles, with the more recent view that it is due to a 
jelly-like ‘‘ultra-clay’’ which surrounds the particles. 
Both conditions exist and exert an influence. The 
‘“‘ultra-clay’’ evidently consists largely of colloidal 
humus, for the smallest particles, with which it ex- 
hibits its main effects, have the largest percentage of 
organic matter. 

Among the natural agencies tending to increase the 
size of the minute soil particles may be mentioned 

73 


74 COLLOID CHEMISTRY 


heat with its drying or evaporative effect, freezing, and 
the coagulating or flocculating action of soluble 
inorganic salts and some organic substances present in 
the soil. On the other hand, included in that little 
known class of substances vaguely described as 
‘‘humus,’’ there are numerous organic substances 
derived from the bacterial, plant, or animal débris, or 
exuded by the roots of plants, which act as protective 
colloids (Schutzkolloide) and tend to produce and 
maintain the hydrosol, or deflocculated condition. 
(See P. Ehrenberg, ‘‘Die Kollide des Ackerbodens,”’ 
Zeits. angew. Chem., 1908, 41, 2122.) In an excellent 
paper on the mechanics of soil moisture, L. J. Briggs 
(U. 8. Dept. of Agric., Bureau of Soils, Bull. No. 10, 
1897) pointed out that very small quantities of certain 
organic substances, such as are continually being 
produced in the soil by the decay of organic matter, 
greatly decrease the surface tension of solutions, thus 
counteracting to a large extent the effects of the surface 
application of soluble salts which would tend to draw 
moisture to the surface by increasing the surface 
tension of the capillary water of soils. It is well known, 
however, that an excess of salts will ruin a soil physi- 
cally, as is evident after flooding by sea water or the 
excessive application of chemical fertilizers. .Of inter- 
est in this connection is the recent work of the Bureau 
of Soils, U. 8. Department of Agriculture, carried out 
by Cameron, Schreiner, Livingston, Gile, Davis, and 
their co-workers. Thus plants grown in the unpro- 
ductive Takoma soil were greatly benefited by green 
manure, oak leaves, tannin and pyrogallol. The 
injurious effects of quinone and some other organic 
substances may be due to their ability to precipitate 


PRACTICAL APPLICATIONS 75 


or flocculate the protective colloids of the soil; for as 
Lumiére and Seyewetz have shown (Bull. Soc. Chim., 
1907, 4, 428-431; J. 8. C. I., 1907, 703), quinone 
renders gelatin insoluble. 

The fact observed by Fickenday (J. Landw., 1906, 
54, 343) that more alkali is required to flocculate 
natural clay soils than kaolin suspensions, he attributes 
to the protective action of the humus present (see 
Keppeler and Spangenberg, J. Landw., 1907, 55, 299). 

Many factors oppose the agriculturally undesirable 
tendency of humus to deflocculate clay, which would 
permit the rain to wash it away: (1) In suitable con- 
centrations humus and clay act as oppositely charged 
colloids, and mutually precipitate each other; (2) 
the lime present in most soils forms with the humus a 
gel of the insoluble calcium humate, which sticks the 
soil particles together. Generally an excess alkalinity 
‘produces humus soils and the evil effects found in 
the heavy, sticky alkali soils. 

When all these factors are properly balanced, the 
soil is in good Zilth,, which Sir E. J. Russell defines as 
that ‘‘nice crumbly condition suitable for a seed bed.”’ 
This permits proper drainage and circulation of air es- 
sential to bacterial activity and plant growth. Humus 
acts as a stabilizer. It makes the soil less sensitive to 
the ruinous coagulating effects of an excess of salts. 
Thus 15 to 20 per cent. of humus will almost obliterate 
the difference between a clay soil and a sandy soil. 
“Humic acid” is a mixture of allied substances 
rather than a definite chemical entity. Its calcium 
‘‘salt”’ exercises a buffer action by combining with 
injurious acids and liberating the harmless, insoluble 
humic acid. 


76 COLLOID CHEMISTRY 


Many important properties of a soil, such as perme- 
ability, capillarity, absorption, moisture content, etc., 
are dependent not so much upon the chemical com- 
position as upon the sizes of its constituent particles. 
In coarse sand, for example, the amount of water is 
greatest at the bottom and smallest at the top, where- 
as in fine clay the distribution is much more uniform. 
Clay consists of particles 0.002 mm. (2 microns) in 
diameter and smaller. Over about 18 per cent. clay 
gives a heavy soil, while under about 4 per cent. means 
a sandy soil of low water-retaining capacity.* The 
effect of lime is so marked that a dressing of calcium 
carbonate will make an 18 per cent. clay soil act like a 
12 to 14 per cent. soil, partly through the formation 
of calcium humate gel, which cements the finer par- 
ticles into the desirable crumbly condition. 

This also makes ploughing much easier; thus in the 
1921-22 report of the Rothampsted Experiment ° 
Station it is stated that chalking heavy soil may re- 
duce the power needed for ploughing by as much as 
15 per cent. 

Apart from the activity of bacteria, earth-worms 
and the like, the movement of substances in the soil 
is effected by diffusion and seepage of soil water. 
The forces acting may be divided into three groups, 
although, of course, there is no sharp line of demarca- 
tion between them: (1) Molecular forces of the order 
of 1,000 atmospheres; (2) capillary forces of the order 
of 3 to 4 atmospheres; (3) gravity. This corresponds 
roughly to the classification of soil water by Bouyou- 
cos, as follows: 


* The application of colloidal clay or bentonite might be useful in 
such cases. 


PRACTICAL APPLICATIONS i 


Gravitational water—“super-available.” 
Free water—very available. 
capillary-absorbed—slightly available. 


Unfree water solid solution 


combined | —unavatbe, 


hydration 
The leaf system of plants is continually evaporating 
moisture, and the resulting hydration and osmotic 
“null” is sufficient to raise a column of water even to 
the top of the tallest sequoia tree. Itis registered as a 
root pull of about 7 to 8 atmospheres. The water 
evaporated by plants is continually replaced by rain, 
but in dry weather there results a struggle for water 
between the plant and the soil. The percentage of 
water left in the soil when the plant can no longer pro- 
tect itself against desiccation by taking up soil mois- 
ture is called the wilting coefficient. Its value varies 
with different plants, and the great influence of particle 
size of the soil may be seen from the following table 
_ of the shrinking coefficient of Kubanka wheat (after 
Briggs & Shantz): 


Wilting 
Soil coefficient 
EDL STEVE lla 2, GE gc a es a Pa 2.59 
am RIR CO Lee ree tose FP eue seis 4 cao aw viecelateiapis alalate als 9.66 
SR aN lee ee a 245s ayo, 4 W dio ale Wain LR ea 16.3 


An important factor in the struggle between the 
“root pull” of the plant and the ‘‘back pull” of the 
soil is the speed with which water can diffuse toward 
the roots. Cultivation of the soil above the roots ex- 
poses a greater soil surface to the air, and the resulting 
increased evaporation there causes increased upward 
and sidewise diffusion streams, which not only supply 
water to the roots at increased speed, but also carry 
the necessary plant food. As Bechhold observed 


78 COLLOID CHEMISTRY 


(Kolloid-Zeit. 27, 29, 1920), soluble salts follow diffusion 
streams. Thus if a piece of plaster of Paris be 
dipped in sulphate of copper solution and then dried, 
the copper will be found almost quantitatively in the 
exterior layers. W. Kraus (Kolloid-Zeit. 28, 161, 1921) 
showed that this ‘‘capillary phenomenon”’ of salt con- 
centration is due to evaporation at the exposed sur- 
faces. The plant is therefore enabled to make its 
necessary growth with less water, and as this growth 
generally is greatest in the wet season, cultivation or 
tillage by losing some water may actually enable the 
plant to enter the dry season with a reserve store of 
soil water in the soil, which in turn is made more 
rapidly available in the dry season by further cultiva- 
tion. (J. Alexander, Science 1921). Cultivation also 
kills off competing weeds and aids soil bacteria which 
produce plant food in available form. 

Contrary to popular idea, colloids do diffuse, albeit 
more slowly than substances molecularly dispersed, 
so that the roots may absorb soil colloids which 
diffuse toward them. While, of course, chemical 
reactions may go on in the soil, ionizable or hydrolyz- 
able salts are apt to be split up and their fractions seg- 
regated by the action of selective adsorption and 
differential diffusion, giving results that cannot be 
simulated by simple aqueous solutions. Changes in 
effective reaction (hydrogen ion concentration) and 
other conditions affect the state of the soil colloids, 
and as their active free surface varies, they may hold or 
release adsorbed substances. Protectors also exert 
their influence. 

In very permeable or sandy soils, especially if de- 
ficient in lime which holds the humus as a gel, the 


PRACTICAL APPLICATIONS 719 


alumina, iron and humus are apt to assume the sol 
form and be washed down by the rain to a lower 
stratum, where, by desiccation or electrolyte precipi- 
tation, they gradually form an insoluble rock-like 
layer known as a pan. Such a pan acts as an imper- 
meable septum, shutting off the soil beneath from air 
and water, preventing diffusion between it and the 
top soil, and often causing swampy conditions. 

A. 8. Cushman [U. 8. Dept. Agri., Bureau of Plant 
Industry, Bull. No. 104] showed that fine grinding of 
feldspar greatly increased the amount of potash avail- 
able under the action of water. A coarse powder 
whose free surface was 43 sq. cm. per cc. yielded only 
0.013 per cent., whereas a fine powder with a free sur- 
face of 501,468 sq. cm. per cc. yielded 0.873 per cent. 
of potash and soda, an increase of 6,700 per cent. 
Even then these fine particles averaged about 0.1 
micron in diameter, which is about the upper limit of 
. colloidal dimensions. In the soil they undergo further 
disintegration whereby still more of their potash be- 
comes available. The availability of phosphate rock 
is greatly increased by fine grinding. 


Clays 


The working properties of clays depend largely upon 
the size and the degree of dispersion and hydration of 
their constituent particles, and especially upon the 
proportion of colloidally dispersed particles. These 
factors are in turn influenced by the presence of pro- 
tective colloids (e.g., humus), salts, and effective 
reaction (p,q, value, hydrogen ion concentration). 
Thus the clay of Mesopotamia is baked into bricks by 
mere exposure to the sun, whereas straw was necessary 


80 - COLLOID CHEMISTRY 


to make bricks out of the clay of Egypt (Exodus V). 
The straw may have been used as a fuel to burn the 
bricks, or else to improve the working quality of the 
clay, for recently patents have been taken out for 
““Eigyptianizing”’ clay by the addition of tannin, ex- 
tracts of straw and humus, ete. Glue and similar 
protective colloids deflocculate or ‘‘free out” clay and 
make it “‘cover”’ when used in paper-coating or kalso- 
mining. 

Bentonites are fine clays resulting from the dusts 
deposited by ancient volcanos. Some varieties form 
suspensions that readily settle, but others are so 
highly colloidal that they swell in water just like glue 
or gelatin. These grades are used for “‘beauty-clays”’ 
and cataplasms, as well as for many technical purposes. 

Clay in general is flocculated by acids and by acid 
or neutral salts. Small amounts of alkalies and of 
alkaline salts deflocculate it, but larger amounts cause 
flocculation. Clay slip, prepared with a little sodium 
hydroxide or carbonate, can be readily poured or cast, 
even though it contains less water than a stiff mass of 
clay and water without alkali. On adding a little acid 
to such fluidified clay slip, the mass immediately be- 
comes so stiff that it will not fall from the inverted 
vessel. 

Hach clay as found in nature bears within itself much 
evidence of its past geological history: the chemical 
and mechanical processes of past ages; lixiviation by 
glacial or other waters; subsequent sedimentation in 
still lakes or flats, or coagulation by saline waters; 
leaching out of some salts or soluble products; infiltra- 
tion of other salts and of organic extractives; the 
pressure of overlying strata; thermal changes due to 


PRACTICAL APPLICATIONS 81 


solar heat, volcanic action, or metamorphic move- 
ments of the earth’s crust, which may also cause 
pressure. 

Ultramicroscopic examination gives a very good 
insight into the nature, working properties and im- 
purities of clays. [See J. Alexander: J. Am. Ceramic 
Soc. 3, 612 (1920).] Some idea can be formed of the 
relative percentage of colloidal and coarser particles, 
and the influence of protectors and coagulating salts 
traced. Fire clays show the combined effect of the 
infiltration of protective substances (from peat, lig- 
nite or coal deposits) and of salts or acids resulting 
from the diffusive decomposition of ferrous sulphate 
(oxidized pyrites). To work properly, clays must 
have the proper proportion of coarser, finer and col- 
loidal particles, so that by washing and blending clays 
and working upon them with reagents more powerful 
than those ordinarily found in nature, we have a wide 
. range of possibilities. 


Ceramics and Refractories 


Following the principles above mentioned, the use of 
alkalis or alkaline salts to deflocculate clay and make a 
casting slip is general in the ceramicindustry. Caustic 
soda and silicate of soda are largely used. If the clay 
mixture holds too much water, there will be excessive 
shrinkage upon drying; and if it is too hydrous, there 
will be too much shrinkage on firing. In either case 
the molded article will warp, and this must be pre- 
vented by proper selection, mixture and treatment of 
the clay used. To safeguard against warping, there is 
often added to the ceramic mixture some powdered 
burnt clay (broken pottery or grog). 


82 COLLOID CHEMISTRY 


When heated or ‘‘ burned,” clays become irreversibly 
dehydrated and harden into pottery, bricks, etc. 
Throughout the whole ceramic process, the pugging, 
blunging and aging of the clay, the molding and sub- 
sequent drying and burning, may be traced the 
influence of particle size and impurities, and their effects 
extend to the zone of actual fusion. 

Fire clays have a preponderance of relatively large, 
consolidated or dehydrated particles, a result which 
may be brought about in nature by pressure, coagulat- 
ing salts and a deficiency of organic deflocculators. 
They take up comparatively little water and therefore 
do not shrink much on drying. With clays exposed 
to high temperatures, the chemical composition, which 
influences the melting point, is vital. 

There may be a ‘‘ eutectic drop” in the case of the 
admixture of clays, just as there is with metallic alloys. 
Mixtures for Seger cones are based on the principles 
above referred to. 

Flotation 

The importance of flotation processes is evident 
from the fact that in 1918 over 70,000,000 tons of ore 
were thus concentrated. The understanding of the 
principles of flotation will be simplified by bearing in 
mind that, notwithstanding the presence of many dis- 
turbing and variable factors, there are involved two 
main triphasic systems: 

1. Ore/water/oil, from which the true ore (usually a 
sulphide) must emerge with a film of oil which enables 
it to be taken up by the air bubbles and thus floated 
off with the froth, while the gangue is wetted and 
flowed off at a lower level. 

2. Air/water/oil, which yields the bubbles to float 
the oiled sulphide. 


PRACTICAL APPLICATIONS 83 


The oil clings to the sulphide because the surface 
tension oil/sulphide is less than the combined surface 
tensions (¢) water/oil and water/sulphide. 


Thatis,cOS <oWO +cWS, 
or cWO > cOS —cWS. 


The oils or some of their constituents distribute them- 
selves at the interface water/air, and thus form stable 
bubbles, because the surface tension water/air exceeds 
the combined surface tensions water/oil and air/oil. 

That is, 


oWA > ocWO + cAO 
75 23 33 (approximate values) 


The air bubbles forming the froth thus have a more 
or less stable surface film which is miscible with, 
though not necessarily identical with, the oil film on 
the sulphide, for selective adsorption may effect some 
_ segregation of the oil constituents. However, the 
oiled ore particles act like oil; they distribute them- 
selves at the interface water/air, are thus attached to 
the air bubbles and lifted to the upper froth layer 
unless they are too heavy or are knocked off. 

Some of the complicating factors will now be con- 
sidered. 

Water.—Dissolved air aids bubble formation by re- 
ducing the internal pressure of the water; so that 
temperature of the water isimportant. Most crystal- 
loids, especially acid electrolytes, act beneficially, for 
they are adsorbed by and coagulate the fine slimes, 
clay, etc., which then tend to repel the flotation oil. 
This recalls the old-fashioned household expedient for 
avoiding the taste of castor oil, by moistening the 


84 COLLOID CHEMISTRY 


mouth and throat with orange or lemon juice. Some 
electrolytes, especially small quantities of alkalies, are 
very detrimental, because by reducing the surface 
tension between the water and the oil and the sulphide 
and the gangue, they tend to emulsify the oil, wet the 
sulphide and deflocculate the slimes. Humic sub- 
stances, especially in the presence of alkali, and pro- 
tective colloids in general, act in like manner; thus 
glue is very injurious. Colloidal clay or very fine 
gangue may have a similar action, for they may act as 
emulsifiers—‘‘fat’”’ clays emulsify oils, tars and as- 
phalts. 

Ore.—Some ores yield soluble substances which 
exert an effect, but as a rule the fineness of grinding is 
the most important factor. While the gangue must 
be fine enough to flow off readily under the conditions 
of operation, it should not be so fine as to cause the 
undesirable emulsification above referred to. The 
sulphide particles, if too large, are not lifted or are 
readily knocked off; if too small the bubble surface is 
covered by too small a weight of ore, and there is great 
loss in efficiency. The tiny ore particles should make 
a substantial armadillo-like covering about the air 
bubbles, which aids in the stabilization of the bubbles. 

Oil.—Many substances used in flotation—e.g., cre- 
sols—are not oils at all, and many substitutes for oil 
have been patented. The rapid development of the 
flotation industry created a sudden demand for 
materials like pine oil which previously had a limited 
demand, and markets were thus turned topsy-turvy. 
Mixtures of substances may be used as ‘‘oil,”’ for the 
foam-producing and the sulphide-oiling factors are 
not necessarily identical. The percentage of oil is 


PRACTICAL APPLICATIONS 85 


also important from an operative as well as from a 
patent standpoint. 

The whole question of flotation demands intensive 
study and experience. Thus Callow estimates that 
with four different oils, three different oil percentages, 
two pulp densities and two temperature changes, 
there are possible about 60,000 different combinations 
of conditions. 


CHAPTER 9 
PRACTICAL APPLICATIONS (Continued) 


DYEING 


Four principal theories of dyeing have held the field: 
(1) The mechanical or physical theory; (2) the 
chemical theory; (8) the colloid-diffusion and ad- 
sorption theory; (4) the electrical theory. A com- 
plete historical review of these theories, with bibliog- 
raphy, is given by P. E. King in the First Report on 
Colloid Chemistry and Its Industrial Applications, 
published by the British Assoc. for the Adv. of Science, 
London, 1917. In the Second Report, London, 1918, 
W. Harrison discusses at some length the colloid chem- 
ical nature of the various textile fibers, as well as the 
electrical theory of dyeing. 

There are many different types of dyes and thou- 
sands of different individual dyestuffs, and the various 
fibers, tissues, etc. (cotton, silk, wool, linen, jute, straw, 
paper, wood) all react characteristically. The pheno- 
mena of dyeing are, therefore, very numerous and com- 
plicated. In essence the conflicting theories are not 
so divergent as might be imagined; they differ mainly 
as to the classification of the attractive force between 
substrate and dye. 

Since all forces of the nature involved are ulti- 
mately due to electro-magnetic fields of the atoms 
involved (or of their electrons), the attraction is essen- 
tially electrical or chemical; but since this attraction is 
exhibited only when the dye and substrate expose suffi- 
cient active free surface, the operation is ‘‘physical” or 
‘“‘colloidal.”? Although forces closely allied to those 

86 


PRACTICAL APPLICATIONS 87 : 


of primary valency are involved, the compounds 
formed do not in general possess that preciseness 
connoted by the present meaning of the expression 
‘‘ chemical compound.”’ 

In some cases the colloid fiber adsorbs the dye, as 
with basic colors which dye silk and wool directly. In 
other cases there is necessary a mordant, which is first 
adsorbed and then fixes the color; e.g., for some dyes 
cotton must first be mordanted with an insoluble col- 
loid precipitate formed in situ by tannin and tartar 
emetic. Certain dyes of opposite charge may mutu- 
ally precipitate each other, and thus serve as mordants 
for each other; e.g., methylene blue and dianil blue 
2R; patent blue V and magenta. But a large excess 
of either dye may serve as a colloidal protector and 
either prevent the precipitate or make it colloidal. 
Griibler’s stains owe their peculiar properties mainly 
to colloidal impurities (dextrin, etc.), so that the same 
dyes when spectroscopically pure, work differently. 

Ultramicroscopic examination reveals the fact that 
many dyes are colloidal in solution, and the selective 
coloring of fibers, tissues, cells, nuclei, etc., probably 
represent the selective adsorption or precipitation 
of one colloid by another, a view sustained by the 
ultramicroscope researches of N. Gaidukov. Some 
dyes can be extracted from the dyed fiber by alcohols, 
which would hardly be the case if a true chemical 
compound were formed. There is an optimum degree 
of dispersion for each dye [zone of maximum colloid- 
ality (see p.17)]. This is shown by the work of R. 
Auerbach [Kolloid Zeit. 34, 109, (1924)] which indi- 
cates that if too coarse the dye lacks diffusive power; 
and if too fine it passes out of the fiber once more. 


88 COLLOID CHEMISTRY 


(J. Alexander, Am. Chem. Soc. Washington Meeting, 
1924.) Temperature, and the addition of salts 
(Na.SOu.), acids, colloidal protectors (boiled off silk 
liquor) and the like are used by practical dyers to 
control the degree of dispersion. 

Colloid chemistry also throws much light upon 
many obscure points in the practical art of dyeing. 
It is possible to obtain much more level colors in old 
dye liquors than in fresh ones, and here it seems that 
colloidally dissolved substances are responsible, exer- 
cising a restraining action upon the absorption of the 
color. The addition of Glaubers’ salt facilitates level 
dyeing, probably by its action as an electrolyte, 
producing a partial coagulation of the dyestuff, so that 
the particles of the latter, thereby made larger, are 
absorbed more slowly and evenly. 

Congo red when dyed on silk or wool changes only 
in shade if the fiber is dipped in dilute sulfuric acid, 
whereas when dyed on cotton it turns blue in like 
circumstances. Upon following ultramicroscopically 
the changes produced in colloidal aqueous solutions 
of benzopurpurin, J. Alexander found [J. Soc. Chem. 
Ind. 1911, Vol. 30, No. 9] that the color change was 
accompanied by an aggregation of the dye ultrami- 
crons. If a protective colloid such as glue was added 
to the dye solution before the addition of acid, the dye 
ultramicrons did not aggregate, and the color change 
could thus be modified or prevented. It seems, then, 
that silk and wool exercise a greater protective action 
on the adsorbed dye than does cotton, which har- 
monizes with the fact that proteins are in general 
better protectors than carbohydrates. This is also 
evidence against the chemical theory of dyeing in this 
case. 


PRACTICAL APPLICATIONS 89 


Shower-proofing Fabrics 


If textile fabrics and the like are coated with a sur- 
face film of wax, aluminium stearate and so on, they 
are not wet by an ordinary shower, whose drops roll 
off ‘‘like water off a duck’s back.”’ With an untreated 
fabric the attraction between the fabric and water 
bursts the surface skin of air on the fabric and 
the surface film of the raindrop, which then spreads 
itself out on the fabric and is absorbed into its pores. 

Remembering that as the attraction between sur- 
faces increases the surface tension between them de- 
creases,* this means that wetting occurs when the 
surface tension (o) water/fabric is less than the com- 
bined surface tension water/air and air/fabric. Thatis, 


oWF < oWA + ocAF,f 
or cWA > oWE — ocAF. 


Since surface tension is only ‘‘skin deep,”’ the sur- 
face tension water/fabric really becomes that of 
water/wax and is greatly increased. Therefore, 


oWE > oWA + ocAP, 
or gWA < oWE — cAP, 


the water is repelled and no wetting occurs. 


Nitrocellulose and Its Products 


When cellulose (cotton, purified paper pulp) is 
soaked in a mixture of nitric and sulphuric acid, it 
takes up from 10 to 13 per cent. of nitrogen, depending 
upon the purity and moisture content of the cellulose, 
the exact composition of the acid bath and the time and 

* In fact, when the surface tension in a two-phase system becomes 


zero, solution or colloidal dispersion may occur. 
+ W = water, F = fabric, A = air, o = surface tension. 


90 COLLOID CHEMISTRY 


temperature of nitration. The formation of a series of 
cellulose nitrates is generally assumed, although A. 
Miiller suggested the existence of adsorption com- 
pounds. In any event the acid fixation curve is con- 
tinuous, notwithstanding numerous attempts to sep- 
arate out definite nitrates. Cellulose has a structure, 
the interior of a cotton fiber being less dense than the 
exterior, and since the nitration starts at the surfaces 
and leaves them comparatively undisturbed, its course 
will evidently be dependent upon the amount of free 
surface exposed. It has been found that by intensive 
grinding cellulose yields a colloidal solution in water. 

The colloidal solutions that result from dissolving 
cellulose nitrate in various solvents or mixtures, are 
widely used as varnishes and lacquers to coat patent 
and artificial leather, metals, etc. Their degree of 
dispersion and therefore their viscosity is dependent 
upon the nature of the solvent, whose action may 
depend upon small percentages of substances which 
exercise a peptizing action. Thus old patents claimed 
that nitrocellulose dissolved in wood alcohol, because 
the ‘‘wood spirit’? of those days contained ketones, 
small quantities of which are essential; for the nitrate 
is insoluble in pure methyl alcohol. According to 
F. Sproxton [Third Report on Colloids, etc., Brit. 
Assoc, Adv. Sci. 1920], if water be added in gradually 
increasing quantities to an acetone solution of cellu- 
lose nitrate, keeping the percentage of the solute con- 
stant, the viscosity rises to a maximum (both abso- 
lutely and relatively to the viscosity of the solvent) 
and then falls until it is practically the same as that of 
the solvent. This seems to be another illustration of 
the zone of maximum degree of colloidality [see J. Alex- 


PRACTICAL APPLICATIONS 91 


ander, J. Am. Chem. Soc. 48, 434, 1921], for the 
Tyndall effect becomes marked close to the position of 
maximum viscosity. 
Celluloid 

This is a transparent gel formed when nitro- 
cellulose is kneaded with alcohol and camphor. In 
the subsequent sheeting, pressing and molding opera- 
tions it undergoes a slow syneresis due to the loss 
of the alcohol and some of the camphor. Schiipphaus 
[Thorpe’s Dict. Appl. Chem., vol. 1] thinks that there 
is some kind of chemical combination between the 
camphor and the nitrocellulose because the heat of 
combustion of the two ingredients is greater than that 
of celluloid itself. This, however, is no criterion, for 
as Prange has shown [Recueil des Trav. Chim. des 
Pays Bas, 9, 121 (1890)] colloids may set free heat if 
their particles aggregate, and heat is liberated when 
nitrocellulose and camphor are mixed. Furthermore, 
according to Sproxton, the optical rotation of a dilute 
acetone solution of celluloid equals the sum of the 
rotations of its two ingredients. 
The view of Dubose [Le Caoutchouc et la Gutta 

Percha, 1919, p. 9803] that celluloid is a camphorgel 
(nitrocellulose dispersed in camphor) seems well 
warranted by the facts. Its plasticity on heating to 
80 deg. C. is attributed to the fusion of the dispersion 
medium, camphor, which diminishes the internal 
friction. The fact that camphor constitutes only 
about one-third the bulk of the celluloid does not 
militate against this view (as Sproxton suggests), 
for S. U. Pickering was able to emulsify 99 per cent. 
of petroleum oil in 1 per cent. of soap solution, obtain- 
ing an almost solid emulsion. 


92 COLLOID CHEMISTRY 


Explosives 

The modern propellant explosives consist of nitro- 
cellulose colloided with ether/alcohol, nitroglycerin, 
etc. The rate of burning of the grain is intimately 
connected with the external and internal surfaces of 
the explosive, the former being controlled by the size, 
shape and perforations of the pieces, and the latter 
by careful regulation of the manufacturing process. 
Variations in the original cellulose, the process of 
manufacture and the presence of traces of impurities 
may materially alter the stability and ballistic proper- 
ties of the product as well as the viscosity of its 
solutions. 

Blasting gelatin is a colloidal gel of 7 to 8 per cent. 
nitrocellulose with nitroglycerine, the former prob- 
ably being the dispersed phase. There is evidently 
an optimum internal structure for this colloid, since 
its tendency to become insensitive on aging is a bane 
of the manufacturer. The use of a protective colloid 
seems indicated here; it will also probably tend to pre- 
vent the exudation of the dangerous nitroglycerine, 
which is especially apt to occur on freezing, and con- 
stitutes a serious danger on thawing or subsequent 
handling. 

Paints, Pigments, Varnishes 

When linseed oil, China wood oil, ete., begin to 
polymerize upon boiling, they form isocolloids, that is, 
a dispersion of the polymerized in the unpolymerized 
oil. Highly boiled China wood oil sets to a gel. The 
various varnish gums go into colloidal dispersion, and 
it seems not improbable that they may act as pro- 
tectors to the boiled oil, keeping it from gelatinizing. 
What is known as the “‘break”’ in boiled oils is the 


PRACTICAL APPLICATIONS 93 


coagulum resulting from the albumin, etc., which 
must be filtered or settled out. 

Varnishes must possess a very fine internal structure 
when dry, for they permit the swelling of gelatin pro- 
tected by the varnish film. In most cases the inter- 
nal spaces permit the formation of a cloudy dispersion 
or emulsion of water in the film, but with the most 
highly protective varnishes they are so fine that al- 
though as much as 5 per cent. of water be taken up, 
the film remains clear. The impermeability of the 
varnish film to most salts indicates the fineness of the 
pores; but since water absorption increases with di- 
minishing salt concentration, it would seem that the 
salt by adsorption or swelling causes a diminution of 
the size of the pores. Normal solutions of NaCl, 
MgCl, and CaCl prevent the ordinary varnish film 
from turning white, and greatly diminishes the amount 
of water passing through. 

Shellac contains about 4 per cent. of wax, which 
goes into colloidal dispersion in most solvents. In 
solutions over about 34 lbs. per gallon the wax does 
not readily settle out, but weaker solutions show a 
separation of wax-free ‘‘French varnish’’ or clear 
shellac solution. 

Pigments vary greatly in their degree of dispersion 
which vitally controls their opacity, surface covering 
power, and oil-absorbing capacity. Because of the 
importance of the physical condition, mere chemical 
analysis is quite insufficient to rate pigments. The 
most finely subdivided oxide of zinc commands the 
highest price, chromates of lead vary in color from 
lemon yellow to orange and Prussian blues vary in 
shades as their particle size changes. The difference 


94 COLLOID CHEMISTRY 


between blanc fixe (precipitated BaSO.) and ground 
barytes is enormous. Too great a degree of dispersion 
is harmful, for there is an optimum degree of dis- 
persion on either side of which desirable properties 
begin to diminish. The cross-precipitation of Bas 
by ZnSO, yields a product so highly colloidal that 
it is furnaced and powdered to produce a more com- 
pact lithopone. The fact that a mixture of blanc 
fixe and zinc sulphide does not give the same degree of 
opacity as lithopone indicates that in the latter there is 
a close adsorption. Generally in making precipitated 
colors or color bases the degree of dispersion is con- 
trolled by temperature, colloidal or other protectors 
(or even coagulants), and the concentrations and 
speed of mixing of the reacting solutions. [See H. A. 
Gardner, J. Ind. Eng. Chem. 8, 794 (1916).] 

While paints are essentially mixtures of pigments, 
drying oils, and solvents, the presence of small amounts 
of other substances may be highly desirable. Thus 
E. G. Acheson [J. Soc. Chem. Ind. 30, 1426 (1921)] 
states that the addition of a minute amount of tannic 
acid causes a pigment to deflocculate readily on grind- 
ing in oil, and the use of about 2 per cent. of water 
emulsified by such a substance as silicate of soda has 
long been common in mixed paints to prevent 
hardening or settling in thecan. Ware and Christman 
[J. Ind. Eng. Chem. 8, 879 (1916)] found that the 
addition of aluminium palmitate or oleate to paints 
helps this condition; these non-aqueous protective 
colloids aid in keeping the pigment in suspension, 
partly because they increase the viscosity of the oil. 
These authors also investigated the ‘‘livering,”’ putty- 
ing, and skinning of mixed paints. Livering is attrib- 


PRACTICAL APPLICATIONS 95 


uted mainly to the formation of a zinc soap gel, the 
formation of which is aided by the presence of zinc 
resinate (formed from rosin used in the paint) and acid 
pigments. 

An interesting application of colloid chemistry 
was made by Wheeler P. Davey, of the General 
Electric Co., who made a ‘‘water japan”’ by emulsi- 
fying japan base (mainly a solution of asphalt in a 
drying oil) in water containing ammonia and a pro- 
tective colloid. Besides being cheaper and less hazard- 
ous than the ordinary japan made with petroleum 
solvents, the water japan is much less viscous and 
small pieces of metal may be coated by first heating 
them and dipping them en masse in baskets into the 
emulsion. As the japanned particles are negatively 
charged, they may also be deposited on the metal 
by making it the anode in an electric circuit. In 
both cases the japan base attached to the metal is 
. free from solvent, and is baked as usual; ‘‘secondary 
drip’”’ and scars due to mutual contact between the 
metal pieces are largely avoided by the wide latitude 
permitted in the composition of the base, for these 
changes do not seriously affect the viscosity of the 
emulsion. 

Emulsions 

An emulsion is a fine dispersion of one liquid in 
another. Since the interfacial tension between benzol 
and water is very high (about 35 dynes per sq. cm.), 
if benzol and water are agitated together, the benzol 
droplets tend quickly to unite, and the liquids separate 
into two layers. If there is a very small percentage 
of benzol, this process takes quite a while, especially if 
the benzol droplets are very small; so that the dilute 
- emulsion has a limited life. 


96 COLLOID CHEMISTRY 


If however the aqueous phase consists of sodium 
oleate solution 0.01 normal or stronger, the inter- 
facial tension is reduced to about two dynes per sq. 
em., and vigorous agitation gives a stable emulsion. 
As J. Willard Gibbs showed, the sodium oleate, 
tending to reduce the interfacial tension, accumulates 
at the interface, and practically surrounds the emul- 
sified droplets with a coherent film. 

Taking Harkin’s view of oriented adsorption, the 
hydrocarbon end of soap will direct itself toward the 
oily liquid, while the metallic end will point toward the 
aqueous phase. If, now, the fatty ends of the mole- 
cule can pack together closer than do the metallic 
ends, the interfacial film bends toward the oil, and we 
have an emulsion of oil in water. This is the case 
with sodium, potassium and cesium soaps; and indeed 
Finkle, Draper and Hildebrand found that the size 
of the oil drops in emulsions, made with soaps of 
these alkali metals, varied as this theory demands 
[J. Am. Chem. Soc., 45, 2780 (1923)]. , 

If, however, a soap of a bi- or trivalent metal 
(calcium, aluminium, magnesium, lead or the like) be 
used, which has several hydrocarbon chains attached, 
then the interfacial film bends towards the water, and 
we have an emulsion of water in oil. Thus are ex- 
plained the observations of G. H. A. Clowes on the 
reversal or inversion of emulsions and the variation 
in the number of drops when oil is dripped into various 
solutions from a staglomometer (drop measurer). 

W. D. Harkins and his collaborators have de- 
veloped the principles governing this oriented wedge 
theory of emulsions and the inversion of emulsions 
[see J. Am. Chem. Soc. 39, 587 and 592-4 (1917); 
also ‘‘Science,’? May 28, 1924]. 


PRACTICAL APPLICATIONS 97 


The preparation of stable emulsions is of importance 
in pharmacy (cod-liver oil, etc.) and cookery (mayon- 
naise, hollandaise sauce, etc.); and the biological 
importance (myelins, nerve tissue, fat depots, etc.) 
is obvious. 

The ‘‘breaking”’ of undesirable emulsions (de-emul- 
sification) is also of considerable practical importance. 
Thus many petroleums contain large amounts of 
emulsified water which must be removed. The basic 
method of F. H. Cottrell [U. 8. P. 287, 115 (1911)] 
makes use of high tension electric current [for details 
see J. Ind. Eng. Chem. 1/3, 1016 (1921)]. Besides 
this filtration, chemical, and centrifugal methods are 
used to break the interfacial films and permit the 
droplets to coalesce. In some cases even as little as 
0.001 per cent. of a protective colloid will do this, 
by coagulating the emulsostatic film. Freezing does 
the same thing in other instances. (For further 
_ details see W. Clayton’s book on ‘‘Emulsions and 
Emulsification,’’ Blakiston, 1923.) 


CHAPTER 10 
PRACTICAL APPLICATIONS (Continued) 
SOAPS 


The orthodox view that soaps are simply alkali salts 
of fatty acids was considerably shaken when Lewko- 
witsch [J. Soc. Chem. Ind. 1907, 26, 590], after epito- 
mizing the views of Merklen, said: 

But whatever may be the outcome of renewed ex- 
periments, Merklen’s views cannot fail to stimulate 
further research into the composition of soap, and 
thus help raise the industry of soap-making, which 
has too long been looked upon as a mere art, to the 
rank of a scientifically well-founded industry, the 
operations of which are governed by the laws of mass 
action, the phase rule, and the modern chemistry of 
colloids. 

Merklen’s views which aroused this comment are 
that commercial soap is a product of essentially 
variable composition which depends on (1) the nature 
of the fatty acids, (2) the composition of the ‘‘nigre”’ 
(in the case of settled soaps), (3) the temperature at 
which the boiling is conducted. It behaves like a 
colloid, and should not be regarded as a compound 
of fatty acids having chemically combined a definite 
amount of water, but is rather an ‘‘adsorption prod- 
uct’ whose composition depends upon the environ- 
ment in which the fatty acid salts find themselves at 
the moment of the finishing operation. 3 

According to J. W. McBain [Third Report on Col- 
loids, etc., Brit. Assoc. Adv. Sci. 1920], homogeneous 
solutions of soap are rarely met with during the process 
of manufacture, since their viscosity would be pro- 

98 


- PRACTICAL APPLICATIONS 99 


hibitively great, and must be held in check by the 
addition of caustic soda, salt, or other electrolytes, to 
bring the soap into a gel or even into coagulated form 
(soap curd). The soap is usually in the form of a dis- 
persion of two soap solutions, soap-in-water, and 
water-in-soap, and may exist in the form sol/sol, 
sol/gel, or sol/curded-gel. 

Most commercial soaps are of the sol/curded-gel 
type, but soft soaps may be of the sol/gel type. 
McBain regards soaps as colloidal electrolytes, that is, 
as salts in which one of the ions has been replaced by 
a heavily charged, heavily hydrated ionic micelle 
which exhibits a high conductivity. 

Martin H. Fischer [‘‘Soaps and Proteins’’] has shown 
that the hydration capacity of soaps varies greatly with 
change in the fatty acid, and the base. Some soaps 
hold enormous amounts of water, while others hold 
very little, so that if the amount of water present ex- 
. ceeds the hydration capacity of the soap, free water 
separates or may remain emulsified (see p. 19). 

Most soaps form crystalloidal solutions in alcohol, 
which ultramicroscopically are quite clear; but if a 
droplet of tincture of green soap, for example, be 
allowed to diffuse into a clear field of pure water held 
between a slide and cover glass in a dark-field con- 
denser, the soap practically explodes into numberless 
actively moving ultramicrons. The colloidal nature 
of aqueous soap solutions is further indicated by their 
turbidity, viscosity, and gelatinization. 

The detergent action of soap is due to its ability as 
a colloid to produce deflocculation. Hillyer [J. Am. 
Chem. Soc. 1903, 25, 511, 1256] showed that this 
action is not due to alkali freed by hydrolysis, and that 


100 COLLOID CHEMISTRY 


alkali itself does not wet oily matter. But all “dirt” 
is not oily, and W. Spring [Kolloid Zeit. 1909, 4, 161; 
6, 11, 104, 164] in experiments with purified lamp- 
black, iron oxide, alumina and silica, showed that 
soap makes such particles less adherent to the fabric, 
and to each other. The attraction of both fabric and 
‘dirt’? for soap exceeds the attraction of the fabric 
for ‘‘dirt”’ and of ‘‘dirt”’ particles for each other. In 
order that the soap may be adsorbed at the interfaces 
involved, it must be in colloidal dispersion, which re- 
quires hot water in the case of many soaps. 

The practical working properties of various com- 
mercial soaps depend mainly upon the nature of the 
fatty acids in them. Thus sodium oleate is much 
more readily dispersible in water to colloidal dimen- 
sions than is sodium stearate, and even small amounts 
of oleate materially assist the higher fatty acid soaps 
to dissolve. In fact mixtures of fats or fatty acids 
show a ‘‘eutectic drop” in melting point, just as do 
alloys. From the oriented wedge theory of emulsions 
(see p. 96) it is evident that in an emulsion of oil in 
water, potassium soaps, having larger molecules (K) 
turned to the water phase than the sodium soaps (Na), 
tend to make a foam with smaller bubbles, which is 
therefore more stable. For this reason shaving soaps 
consist largely of potassium soaps. 

H. Jackson [Cantor Lecture, J. Soc. Arts, 1908, 55, 
1101] examined microscopically the supernatant fluid 
resulting from washing a piece of dirty cloth with soap 
and water, and found in it countless particles in a 
state of oscillatory motion (‘‘pedesis’’). This is 
really Brownian motion, first noted by the English 
botanist Robert Brown in particles approaching the 


PRACTICAL APPLICATIONS 101 


limit of microscopic resolvability. The ultramicro- 
scope shows it to be largely due to bombardment of 
these larger particles by the still smaller and more 
actively moving colloidal particles, which in turn are 
activated by the motion of still smaller particles (water 
molecules), and so on. When an individual fiber was 
bathed in soap solution, the dirt particles gradually 
loosened and began to oscillate. Upon substituting 
salt solution for the soap, the particles flocculated and 
the motion ceased. An ultramicroscopic examina- 
tion of the detergent action of soap is very interesting. 

Transparent soaps are made by keeping the dis- 
persion of the particles of the finished soap well toward 
the lower limit of colloidal dimensions. Among the 
factors that produce this effect are (1) the selection 
of the fatty acids; (2) quick chilling; (3) protective 
colloids; (4) the addition of alcohol, glycerol, sugar 
and the like, which tend to dissolve the soap crystal- 
_loidally. Frequently some of the fatty acid salts 
crystallize out in the clear matrix, marring the commer- 
cial appearance of the soap, and these crystals exhibit 
the dendritic or ramifying tendency common when 
erystallization occurs in the presence of colloids. 
W. D. Richardson [J. Am. Chem. Soc. 30, 1414] 
found that the fatty acids separated from the crystals 
had a higher melting point than those separated from 
the clear matrix. 

To demonstrate the effect of the speed of chilling on 
particle size, J. Alexander melted a piece of commer- 
cial transparent soap and cast it into two cups, one of 
which was instantly chilled with ice, while the other 
was allowed to cool slowly while immersed in hot water. 
The quickly chilled piece was transparent and upon 

8 


102 COLLOID CHEMISTRY 


ultramicroscopic examination showed much smaller 
ultramicrons than the slowly cooled piece, which was 
opaque. After several months standing the quickly 
chilled soap still appeared clear, whereas the other 
had large opaque spots. Upon ultramicroscopic ex- 
amination the transparent piece appeared as before, 
but the slowly cooled piece showed perfectly re- 
solvable crystals in a clear matrix. 


Lubrication 


The automobile has brought such an increase in the 
consumption of petroleum that a material curtailment 
in supply, if not actual exhaustion, lurks in the not far 
distant future. But we may look with confidence to 
colloid chemistry to supply the lubricants of that day. 

The principle involved in lubrication is the main- 
tenance, on each of the surfaces in contact, of an ad- 
sorbed layer of an easily deformable substance (usu- 
ally a fluid) so that the surfaces can not come into 
actual contact, or ‘‘seize.’”? Since the attractive 
forces at surfaces depend on their residual electrical 
fields, lubrication is affected by the chemical and 
physical nature of the surfaces as well as by that of the 
lubricant. Good lubricants are so strongly absorbed 
that great pressure and speed are necessary to tear 
them loose. 

W.B. Hardy [J. Soc. Chem. Ind. 1919, 38, 7 T] 
made some illuminating experiments with glass, which 
absorbs from the atmosphere rather more of its im- 
purities than of the elementary gases, yielding a film 
about 1 millimicron (1 x 1077 cm.) thick, that Lord 
Rayleigh termed ‘‘grease’’ because it has the proper- 
ties of an oil. This ‘‘grease” tends to make pipettes 


PRACTICAL APPLICATIONS 103 


deliver inaccurately, so analytical chemists remove 
the absorbed layer by oxidation with bichromate and 
sulphuric acid. 

For the same reason new or raw glass surfaces have 
mechanical properties different from those of a satis- 
fied or neutral surface. Thus a finger bowl or tumbler 
does not give a musical note when rubbed with the 
wetted finger, unless the ‘“‘grease”’ film is removed by 
vigorous rubbing with the finger tips until a peculiar 
clinging feeling is felt, due to ‘‘seizing’’ between 
finger and glass. 

Experiments with cleaned glass surfaces have shown 
that water, ether, alcohol, benzene and strong am- 
monia do not prevent ‘‘seizing’’ even if the surface be 
flooded with them. A thin film of glycerin did not 
lubricate, but flooding the glass surfaces reduced the 
tangential force required to move them from 55 grams 
to 9 grams. Most acids, for example, sulphuric, 
- acetic, oleic, lubricate better in thick layers. 

Because animal oils, such as sperm or lard oil, are 
more highly adsorbed by metal surfaces than are 
petroleum oils, they have long been blended with the 
latter in considerable quantity to improve the lubri- 
cating value. It has been found, however, that such 
additions owe their value largely to the free fatty 
acids present which are highly adsorbed, and South- 
combe and Wells patented the addition of about 1 per 
cent. of such acid to mineral oils, thereby effecting a 
great saving, by avoiding the cost of the blending oils. 

Most lubricating greases are colloidal oil/water 
emulsions stabilized with sodium or calcium soaps. 
The so-called ‘‘cutting oils’? which form stable emul- 
sions with water are made by mixing petroleum with 


104 COLLOID CHEMISTRY | 


water-soluble protective oils like turkey-red (sul- 
phonated) oil. The thrust boxes of steamers which 
absorb the tremendous pressure of the propellers are 
lubricated and cooled by circulating an emulsion of 
oil and water. 

The viscosity of oils, which is an important factor in 
lubrication, seems to depend to a considerable extent 
on the degree of molecular aggregation. Thus Dun- 
stan and Thole (J. Inst. Petrol. Tech. 1918, 4, 191) 
found that mineral and fatty oils show an exceedingly 
fine heterogeneity in the ultramicroscope which they 
attribute to iso-colloidism. 


Coal 


In recent years the complicated substructure of coal 
has been given much attention. In banded bitu- 
minous coal the following constituents are recognized 
(M. C. Stopes, Proc. Roy. Soc. B, 90, 470 (1919): 


vitrain, brilliant, with conchoidal fracture. 

durain, dull, hard coal. 

clarain, bright coal. 

fusain (‘mother of coal” or ‘‘mineral charcoal’), is 
the most friable portion whose fineness controls the 
speed with which combustion spreads in the mass. 
When very fine it absorbs oxygen with avidity, and 
this may be a factor in spontaneous combustion. 


Stopes and Wheeler (Monograph on Constitution of 
Coal, Brit. Dept. Sci. Ind. Res. 1918) define ordinary 
coal as a compact, stratified mass of mummified 
plants (which have suffered arrested decay to varying 
degrees of completeness), free from all save a very low 
percentage of other matter. Although coal retains to 


PRACTICAL APPLICATIONS 105 


some extent the structure and general chemical nature 
of the original plants, the presence in peat of Doppler- 
ite (a dark, apparently structureless jelly) indicates 
that part of the coal was probably in this state, which 
may account for some of the adsorbed substances found 
in coal. 

Reinhardt Thiessen (U. S. Bureau of Mines, Bull. 
117; Journ. of Geology, 28, 185-208, 1920), divides 
the main constituents of coal into (1) bright or glanz 
coal, (antharyxalon) compact, pitchy jet-black, and of 
conchoidal fracture, formed from undisintegrated 
trunks and large limbs and retaining the original 
woody structure, and (2) dull or mat coal, less com- 
pact, dull grayish, and showing an irregular fracture; 
consisting of numerous small layers or chips of an- 
thraxylon embedded in a dull matrix, the atiritus, 
which is derived from assorted vegetable residues. 

In the very nature of things, classifications of coal 
- constituents must be rather arbitrary, but it is evident 
that the sub-microscopic structure of coal has much 
to do with its practical working properties. 


‘6 Colloidal ’? Fuel 


“Colloidal”? fuel consists of finely powdered coal, 
cheap tars and the like, dispersed in mineral oil and 
stabilized by a protective colloid or ‘“‘fixateur”’ such 
as lime soaps or resinates, so that it may be stored, 
piped, atomized and burned practically like oil itself. 
As this new composite fuel will at one stroke relieve 
the drain on the earth’s rapidly diminishing stores 
of petroleum, and lead, as well, to the utilization of 
coal waste (culm, screenings), inferior fuels (peat, 
lignite), and even cellulose waste (slabs, sawdust), 


106 COLLOID CHEMISTRY 


it may become a material factor in the conservation 
of our natural resources. | 

Coal or other combustible solid is prepared for dis- 
persion by being pulverized so that about 95 per cent. 
passes through a 100-mesh screen, and 85 per cent. 
through a 200-mesh screen. This means, of course, 
that the bulk of the weight is in particles hundreds 
and thousands of times larger than colloidal dimen- 
sions. But the violent motion of colloidal particles 
aids in maintaining the Brownian motion of larger 
particles which helps to keep them afloat. A fluid 
fuel may be made containing as much as 40 per cent. 
by weight of powdered coal, and mobile pastes con- 
taining up to about 75 per cent. Mobile gels may 
be made from either liquid or paste. 

- Ordinarily between 0.5 and 1.5 per cent. of rosin or 
a saponifiable oil is used, and 0.1 per cent. exercises a 
noticeable effect. The amount is determined by the 
nature of the mixture and the degrees of permanence 
desired. The bulk of the particles does not begin to 
settle until the period of ‘“‘life’ has passed, the 
colloidal fuel having a limited ‘‘life’’ which may be 
regulated to meet requirements—days for power 
plants, weeks and months for central storage stations 
and ocean-going vessels. Heat and agitation re- 
vivify the liquid fuel; the paste form may be kept for 
years. 

‘‘Colloidal”’ fuel is heavier than oil or water and 
saves storage space by compressing a maximum ther- 
mal value per unit of space. It may be stored under 
water to avoid evaporation, deterioration and fire 
risk. When sprayed into the hot fire-box, its oil- 
soaked solid particles are still further atomized by 


PRACTICAL APPLICATIONS 107 


the sudden gasification of their imbibed oil. It 
possesses the advantages of fuel oil over coal in 
absence of smoke, dust and ash, practical elimination 
of labor in firing and filling storage space, thus saving 
time in ‘‘coaling”’ and in raising steam. The fire is 
likewise subject to instant control, and in naval opera- 
tions a smoke screen may be readily produced by over- 
firing. : 

Another well-known colloidal fuel is ‘‘solidified 
alcohol,’’ an alcogel usually stabilized by an alcohol- 
soluble protector. ‘Thus the late Prof. Charles Bas- 
kerville found that certain percentages of calcium ace- 
tate yielded with alcohol an unstable gel, which is 
stabilized by stearic acid. 


Petroleum 


Colloidal phenomena are very marked in mineral 
oils. Many oils contain colloidally dispersed water 
which is difficult to remove. Cottrell devised a 
method of removing it with high tension current. He 
also was able to send viscous asphaltic oils through 
pipe-lines by emulsifying in them a certain percentage 
of water. ‘The solid constituents of petroleum, espe- 
cially the waxes, are largely in colloidal dispersion, 
some apparently serving as protectors to the others.* 
This phenomenon is common, being found in alloys, 
glasses, mixtures of waxes, fatty acids, etc. When the 
kerosene and lower boiling fractions are removed 
from such oils, the solids, on chilling with ice, often 
assume the form of a gel which may be broken up by 
stirring or shaking, somewhat like agar gel. 

Bloom in oil seems to be in part caused by some col- 
loidal constituent. 

* See e. g. H. V. Dodd, Chem. Met. Eng., 28, 249, (1923). 


108 COLLOID CHEMISTRY 


As Brooks and Humphrey have shown [J. Am. Chem. 
Soc. 40, 882 (1920)], the ‘‘acid tar’? which forms when 
petroleum is refined with sulphuric acid represents 
the coagulation of a previously existing colloidal dis- 
persion of asphaltic bodies and the like, and not a 
polymerization product of olefines. Even 100 per 
cent. acid at 15 deg. C. does not produce tars from 
pure olefines. In some cases impurities may be cen- 
trifuged out, but they are largely removed by ab- 
sorption with fuller’s earth, floridin, bentonite, bauxite 
or other adsorbents. Patrick’s colloidal silica gel and 
activated carbons are now used to adsorb the pe- 
troleum dispersed in natural gas, which yields millions 
of gallons annually of the high-grade ‘‘casing head”’ 
gasoline. 

Great care must be used in drilling oil wells, for if 
the oil sands below ground get wet with water, surface 
tension makes the sand repel the oil, which is then 
unable to diffuse toward the well-pipe. Once the 
sands are ovled, however, then water cannot wet them. 

In oil shale the oil is so finely dispersed that it can- 
not be removed by mere pressure. Distillation is 
resorted to, though doubtful claims are made that 
grinding in the colloid mill will effect a separation. 


Asphalt 


Although most of the mineral matter in asphalt 
exceeds colloidal dimensions, as Clifford Richardson 
has shown, the great extension of free surface caused 
by its presence adds considerably to the viscosity, 
strength and wearing properties of the asphalt. The 
minerals here function somewhat like the fillers in 
putties and lutes. Natural asphalts may be improved 
by the addition of colloidal clays. 


PRACTICAL APPLICATIONS 109 


L. Kirschbraun has patented a colloidal aqueous 
dispersion of asphalt, pitch, etc., which, when mixed 
with paper stock in the beater or on the way to the 
paper machine, may be flocculated out with alum or 
alum and sodium silicate, giving a highly waterproof 
and moisture-resisting paper. This asphalt dispersion 
is not sticky or adhesive under the conditions of 
pressure, etc., in the paper machine. 

Among the many uses to which such emulsions are 
being put are protecting structural steel, patching 
roofs and highways, making waterproof cork insula- 
tion, pipe coverings, paints, building and roofing 
papers, auto body parts, and especially making a 
large variety of moisture and waterproof containers 
and wrappers for foods, tobacco, etc. 


‘¢ Fire-Foam ”’ 


‘“‘Fire-Foam”’ is a carbon dioxide froth made by 
mixing solutions of sodium bicarbonate and alum. 
A protective colloid such as liquorice, glue, dextrin, or 
saponin, is added to stabilize the foam and keep the 
bubbles in finely dispersed form. 


Insecticides 


Petroleum oils are readily emulsified in water for 
killing San Jose scale and similar parasites, by the 
addition to them of sulphonated oils. Sulphur, 
Bordeaux mixture and the like are maintained in fine 
dispersion by colloidal protectors, as are also such 
poisons as lead arsenate. Minute quantities of poisons 
are sufficient to kill insects and it is wasteful to supply 
more than the necessary dose. The colloidal poison 
is less apt to be noticed and rejected by the insect and 
clings better to the plant, and goes much further. 


110 COLLOID CHEMISTRY 


Even allowing for the necessity of a rather promis- 
cuous distribution of insecticides, in general the use of 
coarse particles means that the factor of safety is 
exceeded hundreds if not thousands of times; and this 
means enormous economic loss. 

The electric charge of the dispersed insecticide is of 
_ importance. Since the leaves of plants are mostly 
negatively charged, positively charged colloids tend 
to cling closely to them, as has been pointed out by 
Dr. William Moore. 


CHAPTER 11 
PRACTICAL APPLICATIONS (Continued) 
FILTRATION 


To secure commercially successful filtration, a filter- 
ing medium or septum must be used, which, under the 
application of a practicable pressure, will permit the 
fluid to pass and still hold back the solids. The pores 
of the filter may initially be small enough, or they 
may become so by the deposit of some of the precipi- 
tate itself or of some added substance, such as paper 
pulp, shredded asbestos (Gooch crucible), or the like. 
Exceedingly dense filters may require the application 
of high pressure to force the fluid through, and such 
high-pressure filters were termed by Bechhold wléra- 
filters because they could be made to hold back 
ultramicroscopic particles. Bechhold (‘‘ Colloids in Bi- 
ology and Medicine,” translated by Dr. J. G. M. 
Bullowa) used gelatin jellies hardened in ice-cold 
formaldehyde, or a glacial acetic acid solution of col- 
lodion coagulated in water. These septa were sup- 
ported on fine-meshed wire cloth to withstand the high 
pressure used. 

Zsigmondy has recently prepared wltrafilters for an- 
alytical use, which permit the rapid and successful 
filtration of troublesome precipitates [see Zsigmondy 
and Jander, Zeit. anal. Chem. 58, 241 (1919)]. 

The so-called séream-line filter of Hele-Shaw con- 
sists of paper sheets tightly pressed together, with holes 
bored through the pack at right angles to the surface. 
Liquids are forced under pressure into one set of holes 


and pass between the sheets to the adjoining set. By 
111 


1 Aa Re COLLOID CHEMISTRY 


sufficient compression of the sheets, even Prussian 
blue may be filtered out of solution. 

Anything that favors deflocculation of a precipitate 
or the formation of a fine or colloidal precipitate, such 
as protective colloids, dilute alkalis, and so on, works 
against clear filtration. Thus Zsigmondy found that 
a gold hydrosol, whose particles were 20 to 30 milli- 
microns, passed freely through Pukall and Maassen 
filters if egg albumen was present. In the absence 
of the protective albumen, the colloidal gold was ab- 
sorbed by the filters, gradually clogging their pores 
until the filtrate, at first red, became colorless. 

In technical practice, wherever possible, a coagu- 
lated precipitate is formed whose large particles are 
held back with comparative ease. Therefore, glucose 
liquors, for example, must be acid to filter well. 
Where protective colloids interfere with filtration, 
their protective action may be destroyed by coagula- 
tion, hydrolysis and the like. 


Sewage Disposal 


Sewage usually contains considerable amounts of 
deflocculating substances, such as soap, organic matter 
and the like, which tend to keep it full of colloidally 
dispersed particles. The successful treatment of 
sewage, backwaters, and trade effluents involves the 
separation from them of these colloids by coagulation, 
adsorption, filtration, or destruction (usually by bac- 
teria in the septic tank). The old ‘‘ABC” method 
depended on the use of alum, blood, and clay (whence 
the name) to make a coagulum that would carry down 
suspended matter. Ferrous sulphate and lime (yield- 
ing a coagulum of ferric hydroxide) and alum are 


PRACTICAL APPLICATIONS 13 


also used as clarifiers and coagulants. H.M. Spencer 
(Chem. Age, Jan. 1924) uses a colloidal solution of 
aluminum hydroxide, which on dilution forms a gel. 

In the activated sludge process it has long been known 
that the effective acidity of the sewage controls proper 
settling, and modern methods of py control aid in 
fixing the isoelectric point, most favorable to floccu- 
lation. 

Filtration through sand, coke, and the like serves 
to adsorb colloidal impurities, but filters of this kind 
do not begin to function properly until they become 
coated with a gel-like slime containing many bacteria. 
There has been much discussion as to whether the 
action of the filter depends on activity of the bacteria 
in destroying and dispersing to the crystalloidal state 
the finely suspended or colloidal matter, or whether 
the removal of these substances is not an adsorption 
phenomenon due to the extended surface of the filter 
bed. Both effects exist, the latter being the more 
important, and depending largely on the action of the 
bacterial jelly that forms the surface layer. 


Photography 

The ordinary precipitate of silver bromide obtained 
by mixing equivalent amounts of potassium bromide 
and silver nitrate is flocculent and quickly settles. 
If the fresh precipitate (before or after washing) is 
shaken with a solution of gelatin containing a trace 
of potassium bromide, or better, yet, if the precipitate 
is originally formed in the presence of gelatin, there 
results a colloidal solution of silver bromide, which 
may be exceedingly fine or grainless, depending on 
conditions. The latter method is used mainly in 
preparing photographic ‘‘emulsions.” 


114 COLLOID CHEMISTRY 


The Lippmann “‘grainless”’ emulsion is an illustra- 
tion of the principle of plural protection discovered 
by J. Alexander [J. Chem. Ind. Eng. 15, 283 (1923)], 
for in preparing it gelatin is added to both the silver 
nitrate and the bromide solutions before they are 
mixed (see p. 43). 

As a very fine emulsion is relatively insensitive, 
it is ‘“‘ripened’’ by allowing the silver bromide grains 
to grow until they are about 0.4 to 10 microns in 
diameter, the process being facilitated by heat. 
Certain percentages of chloride and iodide may be 
used, a small excess of the soluble halide salt aiding 
in peptizing or stabilizing the precipitate, and keeping 
its particles within the desired limits which are about 
2 to 3 microns in most cases.” 

When a sensitive ripened emulsion is exposed to 
light, some of the silver halide is decomposed, its 
halogen being probably held by the gelatin,t while 
the metallic silver forms a colloidal dispersion in the 
rest of the silver halide, yielding the so-called 
‘‘nhoto-halides.” These were long supposed to be 
subhalides (sub-bromide and so on) of silver, but are 
now recognized as being of the same nature as the 
“metal fogs’ which often appear during the elec- 
trolysis of molten salts, and consist of colloidally dis- 
persed metal. 

The particles of silver in the “latent image”’ are so 
exceedingly fine that, in time, they apparently recom- 


* According to Lottermoser the excess of soluble halide gives the 
particles of silver halide a positive charge, essential to their sensitivity 
to light. 

+ Chlorine, bromine and iodine all “tan gelatin”; see Allen’s Com- 
mercial Organic Analysis, 4th ed., vol. 8, article on Albuminoids or 
Scleroproteins; also A. C. S. Monograph No. 11, ‘‘Glue and Gelatin,” 
by J. Alexander. 


PRACTICAL APPLICATIONS 115 


bine again with the halogen, and the latent image 
fades away. To render it permanent, it must be 
treated with a ‘‘developer,”’ a reducing agent which 
probably serves the twofold purpose of combining 
with the halogen and making the extremely fine silver 
grow into dense visible particles or grains. 

The color of emulsions varies greatly with the size 
of their ‘“‘grains.”’ Fine blue emulsions are sensitive 
to red, and even to infra-red rays. Freshly prepared 
emulsions appear red to transmitted light, but on 
ripening the color changes to green, the particles in- 
creasing from below 0.1 micron to over 0.5 micron. 
The photo-halides resulting from the action of light 
on emulsions are also highly colored. 


Brewing 


In the brewing process it is essential that the starch 
originally present be converted into sugars which can 
' be fermented, or into very soluble dextrins. In fact 
both the albumin and the dextrin in beer must be 
finely enough dispersed to remain clear in solution. 
Many factors in the brewing process may tend to 
coagulate the albumin. The influence of solid sur- 
faces is seen by changing the inner lining of the fer- 
menting vessel. A certain wort, which showed 0.2450 
per cent. of albumin when fermented in a glass or 
enameled vessel, showed only 0.1925 per cent. in a 
paraffin-lined vessel, and 0.1750 per cent. in one lined 
with pitch. 

Old-fashioned brew-masters would never use any 
vessel unless it had first been treated with a decoction 
of malt kernels and nut leaves, or else with “‘fassge- 
lager” (barrel dregs) which acts like the so-called 


116 COLLOID CHEMISTRY 


‘“‘bierstein,’’ a deposit mainly of organic substances 
that forms on new surfaces, and prevents them from 
coagulating albumin. 

Fluid surfaces also exercise an effect as may be seen 
from the fact that in the chemical analysis of beer, 
such substances as benzol, chloroform and benzin are 
used to coagulate and shake out the beer colloids. 

Gas surfaces are also active. Their formation tends 
to coagulate the dissolved albumin, and this fact killed 
the so-called ‘‘vacuum fermentation process” de- 
signed to recover and use the carbon dioxide liberated 
on fermentation. Jarring due to transportation or 
passing railroad trains may have a deleterious effect. 
A slight trace of acid tends to stabilize the albumin, 
as do also the tannin and resin from the hops, the 
dextrins from the mash, and the inorganic colloids 
of calcium and magnesium. The excellent brewing 
qualities of the water of the River Trent (England) 
are in large measure due to the lime content of the 
water, and patents have been taken out for ‘“‘ Burton- 
izing’ water by the addition of lime salts. 

To secure the formation of a lasting foam and a 
desirable ‘‘body’”’ (Vollmiindigkeit), a proper balance 
is necessary between dextrin and albumin. Where 
beer is served icy cold, the chilling produces cloudi- 
ness because of the aggregation or partial coagulation 
of the albumin. This was cleverly overcome by 
Wallerstein, who introduced into the beer a proteolytic 
enzyme (pepsin or papéin) which, by increasing the 
dispersion of the albumin, prevents clouding. 

Isinglass and gelatin are largely used for fining 
beer and astringent wines. A solution of the fining 
agent is poured in through the bung and settles down 


PRACTICAL APPLICATIONS 117 


through the cask as a gel which effectually entraps 
all turbidity-producing particles. The tannin present 
aids in coagulating the gelatin or isinglass. 


Tanning 


The skins or hides of animals consist of an organized 
colloidal jelly, formed of bundles of fine fibrils about 
one micron in diameter, bound together by cementing 
substance of similar chemical composition which is 
largely removed by liming and other treatment pre- 
ceding the tannage proper. ‘The principal protein of 
the hide is called collagen (glue-former) since from it 
gelatin and glue may be produced by heating with 
water. The old processes of bateing, drenching, 
puering, etc., tended to neutralize the lime and bring 
the collagen into a flaccid or unswollen condition; in 
addition, the bacterial enzymes digest off part of the 
remaining cementing and epidermal substances (elastin) 
. and complete the emulsification of the fat. Synthetic 
mixtures consisting mainly of ammonium chloride and 
tryptic ferments (pancretin) are now largely used in 
place of the foul-smelling bacterial baths which often 
ate off the grain of the skin. 

Much work remains to be done to trace out the 
action of tanning processes and chemicals on the 
three colloidal elements in pelt: 

1. The primary colloidal particles of the proteins. 

2. The secondary colloidal particles of the proteins. 

3. The fibrils of collagen (which are about 1 uw in 
diameter) and other organized structures. 

Tanners well know the diversity of effects that 
may be produced in the finished leather by varying 
the successive treatments before actual tanning begins; 
pre cannot reach a scientific understanding of the 


118 COLLOID CHEMISTRY 


processes at work until we separate the influence of 
the several factors on each of the main colloidal 
elements above referred to. 

In vegetable tannage, the prepared hide is placed 
in an acid tannin solution (tan liquor), whereupon the 
hide powerfully adsorbs tannin and combines with 
it to form leather. In alkaline solution both tannin 
and hide are negatively charged, and no tanning oc- 
curs; in fact, leather may be stripped of tannin by 
alkalis. It is still a moot question whether the com- 
bination is chemical or physical, but since the fixation 
follows an adsorption isotherm and is very variable 
in percentage, it may be justly termed a colloid com- 
bination which partakes of the nature of both, but is 
chiefly physical rather than chemical—if we use these 
terms in their established meanings. The positively 
charged hide and the negatively charged tannin 
mutually coagulate each other. Neutral gelatin, if 
practically free from electrolytes, does not precipitate 
pure tannin, but in acid solution it takes a positive 
charge and is tanned. The tanning process may be 
aided by giving the hide a suitable potential, positive 
in the case of vegetable tannage, negative with chrome. 

To prevent the skin from being ‘‘case-hardened”’ 
by too rapid a deposition of tannin, which would 
make it act as an ultrafilter and prevent proper tan- 
nage of the interior layers, the tanning is usually 
commenced in very weak or spent liquors, and the 
skins are moved to baths of progressively increasing 
strength. Non-tannins seem to play the role of 
protectors to the tannins, and the general rule is that 
tanning liquors having the highest tanning/non-tannin 
ratio are the most astringent. The degree of dis- 


PRACTICAL APPLICATIONS 119 


persion thus appears to be an important factor; for 
if highly dispersed, tannin easily enters the tiny inter- 
spaces between the fibrils. The efficiency of certain 
synthetic tanning agents used preliminary to heavy 
vegetable tannage may be consequent to their pro- 
tective action, or their ability to enlarge the inter- 
spaces by shrinking the fibrils, thus allowing the tannin 
to enter the hide more rapidly and speed the tannage. 
[See J. Alexander, J. Am. Leather Chem. Assn., Vol. 
18, p. 400, 1923.]. 

The degree of dispersion of the tannin and the inci- 
dental impurities present also influence the intimacy 
with which the tannin is fixed by the hide. By ex- 
traction with organic solvents G. Powarnin (Col- 
legium, 1923, 222-228) obtained the following results 
from a quebracho tanned leather which showed a cold 
water extract of 9.54 per cent. by the Koch-Procter 
method: water-soluble material removed upon Soxhlet 
extraction by acetone, 12.5 per cent.; by ethyl alcohol, 
17.2 per cent.; by methyl alcohol, 22.2 per cent. His 
tabulated results indicate that for every different tan- 
ning material used, part combines chemically and part 
is adsorbed. 

Chrome and alum are believed by H. R. Procter 
[First Report on Colloid Chemistry, etc., British 
Assoc. Adv. Science, 1917] to be fixed by the hide in 
the form of basic salts, so that initially the tannage is 
largely physical. J. A. Wilson, among others, thinks 
that a definite chemical combination occurs subse- 
quently. The whole process, however, seems to be a 
hydrolytic splitting of the salts, accompanied by ad- 
sorption (or sorption, to use McBain’s non-committal 
expression), of the insolublé basic metallic salt by the 


120 COLLOID CHEMISTRY 


hide fiber. The increasing stability of alum-tanned 
leather upon aging would, then, be due to colloidal 
syneresis. 

Fat tannage, which yields chamois, buckskin, 
washleather, and the like, is effected by introducing 
oxidizable oils, usually in the form of a colloidal emul- 
sion, into the skin. The view that fat tannage is due 
to aldehydes, developed by oxidization of the glycerin 
in the fats used, seems negatived by the fact that free 
fatty acids themselves may serve as tanning agents. 
Apparently the colloidal fat and the hide, being oppo- 
sitely charged, form an adsorption compound Knapp 
made a fair grade of leather by simply soaking a flaccid 
skin in an alcoholic solution of stearic acid. 


Paper 


The first paper maker was the hornet or wasp, who, 
by chewing up wood in the presence of his colloidal 
salivary secretion, imitated on a small scale the action 
of the ‘“‘colloid mill.’”’ There is no doubt that the 
prolonged and thorough grinding that paper stock 
gets in the beater, besides bringing much of it to a 
swollen or gel-like state, reduces part at least to a 
colloidal condition This colloidal cellulose evidently 
exerts a powerful cementing action, far above what 
would be indicated by the small percentage present 
by weight, resembling in this respect the “‘ultra- 
clay”? which exerts so great an effect in soils. The 
formation of a coherent web on the fourdriner wires 
thus depends on the greatly extended free surface of 
the ‘‘stuff”’ or paper stock, and perhaps the addition 
of a little colloidal cellulose will materially shorten the 
timein the beater. ‘‘Onion-skin”’ papers are produced 
by excessive beating. 


PRACTICAL APPLICATIONS P21 


In engine sizing a colloidal emulsion of rosin in rosin 
soap is generally added to the beater pulp, and, after 
being thoroughly mixed in, is precipitated by adding 
alum or aluminium sulphate. The colloidal rosin and 
the alumina gel help bind the paper fibers together 
and to make the paper stronger and resistant to water. 
Many substitutes for rosin have been used (silicate 
of soda, glue, and so on) which are added in colloidal 
solution and subsequently precipitated. To prevent 
the glue from being largely lost in the back waters, 
some highly adsorbent filler such as tale or clay must 
be added to the pulp to adsorb and hold the glue. 

Tub sizing of paper involves passing the finished 
web through a bath of some such colloidal solution as 
gelatin, glue, starch, and the like, which, on drying, 
fills the pores and prevents ink from spreading or 
‘‘bleeding,”’ besides adding to the finish, strength and 
‘“‘handle’’ of the paper. The nice, crisp crackle of 
new banknotes and of stocks and bonds is due to a 
bath of alum and glue. 

Bibulous papers, such as blotting paper and filter 
paper, are made without sizing, reliance being placed 
for their strength on the binding properties of the col- 
loidal cellulose. Recently rubber latex has been tried 
as & paper size. 

The speed and the degree of drying paper are also 
factors materially affecting its strength. 

Colloidal clays have found several uses in the paper 
industry. They tend to improve the luster of friction 
coated papers. Asphalts or tars may be emulsified 
with water by their aid, and incorporated with the 
paper pulp in the beater, thus serving both to size and 
to waterproof the paper. Old newspapers and maga- 


122 COLLOID CHEMISTRY 


zines may be de-inked by beating and macerating them 
with a little alkali and colloidal clay, which adsorbs 
the loosened ink so that the two may be washed away 
through a special screen. It is estimated that a ton 
of good standard newspaper can be produced from a 
ton and a quarter of old newspapers, and by gathering 
the waste in big cities alone there would be possible 
an annual saving equal to the cut on 275,000 acres of 
thickly wooded spruce forest. (Forest Products Labo- 
ratory, Madison, Wis.) 


Rubber 


The milky juice (latex) of several kinds of plants 
yields on coagulation the colloidal gel called rubber. 
The latices are emulsions containing rubber and resin, 
stabilized by protective colloids (proteins or peptones) 
and the kind of coagulant used depends on the nature 
of the protector. Thus formaldehyde preserves latices 
whose protectors are proteins, but coagulates kickexia 
latex by precipitating the protective peptones. 

Vulcanization consists in the combination of sulphur 
with rubber. The sulphur is at first adsorbed, and 
upon heating enters into a very close combination 
with the rubber, which is generally believed to be a 
true chemical combination. 

Fillers increase considerably the strength and 
wearing power of rubber (in automobile tires, for 
example) by greatly extending the free internal sur- 
face. The réle of fillers in rubber is similar to what it 
is in putty, and is therefore intimately bound up with 
their degree of dispersion. Carbon black, zine oxide 
and magnesia are valued in the order given, which 
represents the relative order of fineness of their par- 


PRACTICAL APPLICATIONS 123 


ticles. An especially fine zinc oxide has recently been 
produced which is said to give the tread of automobile 
tires twice the life of those made with ordinary zinc 
oxide. 

Patents have been taken out for incorporating col- 
loidal precipitates in rubber, and even glue jelly is 
worked into tires to improve their wear by increasing 
the interior free surface. 

Rubber latex is now being largely imported and 
used for directly impregnating fabrics which are then 
vulcanized, and also for adding to paper pulp in the 
beater. Tough, waterproof paper may be thus made. 

Wm. B. Pratt has recently patented a process for 
dispersing commercial rubber (coagulated latex) in 
water (Italian Pat. 225,949, Dec. 12, 1923). Be- 
lieving that the original globules in latex maintain 
their identity after coagulation and even after vul- 
canization, he incorporated protective colloids (soaps, 
glue) with the rubber, or formed them in situ, and 
then gradually worked in water. The rubber may 
first be swollen with benzol to open it up. As the 
coagulation of rubber was thought to be irreversible, 
this is practically unscrambling an egg. (See J. B. 
Tuttle, India Rubber World, Jan. 1924; Chem. Met. 
Eng., March 10, 1924.) 


CHAPTER 12 
PRACTICAL APPLICATIONS (Continued) 
FOODS AND THEIR PREPARATION 


It is a serious error to judge foods on the basis of a 
bald chemical or calorific analysis. Fat, protein, 
carbohydrate and calories alone are not fair criteria 
of food value; the physical condition of the food and 
the presence of certain ‘‘impurities’’ now recognized 
as essential (for example, vitamines) largely affect the 
usefulness of a food to the organism. The experience 
of centuries has taught us the value of cooking which 
hydrolyzes, swells up, or softens many foods and 
usually destroys the species-specificity of proteins. 
The Chinese sprout many grains before cooking them. — 
‘“‘Light”’ bread or cake has been leavened by yeast or 
baking powder until it presents an enormous surface 
to the digestive juices; as we learn from the Bible, 
unleavened bread was eaten only in times of stress. 
EKgg-albumen, when cooked, is more slowly absorbed 
and its coagulum is reduced to its constituent amino- 
acids which are absorbed as such. Therefore, some 
persons who have an idiosyncrasy against raw eggs 
(possibly because of a permeable or ulcerated stomach 
wall) can eat cooked eggs. 

The meats yielded by young animals are more juicy 
and tender than those obtained from older animals, 
because the latter consist of tissues which age has 
hardened by syneresis and dehydration. In cooking 
meats, the exterior is seared to retain the juices (e.g. 
pot roast); but in cooking soup it is simmered to 
extract them. Many foods are highly emulsified 
(mayonnaise, for example), and this permits their more 

124 


PRACTICAL APPLICATIONS 125 


ready assimilation. Cream is a natural emulsion of 
fat in water and wets blotting paper; butter is an 
emulsion of water in fat and greases paper. The 
presence of protective colloids in foods containing 
milk (ice creams, Bavarian creams and the like) is an 
aid to their digestibility. 

The ancient art of cooking, however, involves psy- 
chological factors besides mere digestibility and ease 
of assimilation; taste, flavor, odor and variety are 
important. 

From time immemorial many colloid chemical 
principles have been utilized in the kitchen; e.g., the 
making of Hollandaise sauce, mayonnaise, clarifying 
coffee by adding egg shells containing some adherent 
albumen, salting water to ‘“‘set’’ eggs in poaching. 
Agar, an indigestible carbohydrate, forms a hydrous 
residue stimulating to the intestines. 

Oysters, scallops ,etc., are often allowed to “ drink” 
fresh or brackish water, which removes repressing 
salts and permits swelling or plumping. In making 
‘‘cream of tomato,” it is usual to add some baking 
soda (bicarbonate)to prevent the acid fruit from co- 
agulating the milk. 

Baking 

In China, 2000 B. C., the art of baking bread was 
already an ancient one, and probably developed from 
the observation that fermentation of the dough by wild 
yeasts improved the bread. Bread involves a rather 
complicated colloidal system consisting of starch, 
gluten, salts, yeast enzymes, and often fat and milk. 
That the system must contain stabilizing factors is 
indicated by the fact that almost anyone, working by 
rule-of-thumb methods, can produce a fair loaf. 


126 COLLOID CHEMISTRY 


Although the temperature of a bread oven usually 
runs between 200 deg. and 240 deg. C., the interior of 
the loaf seldom goes over 95 deg. C. Wheat starch is 
completely gelatinized at 65 deg. C. if sufficient water 
be present; but as the average dough holds only about 
40 per cent. of water, a large part of which is taken 
up by the gluten, the baked loaf is full of starch granules 
that have been only partially swollen and gelatinized. 
According to R. Whymper (‘‘The Conditions That 
Govern Staleness in Bread,’ Maclaren & Sons, Ltd., 
1919), the gluten acts as a colloidal protector to the 
starch, maintaining its dispersion and water-holding 
capacity. 

The gluten of wheat flour and like grains is itself a 
colloidal complex and of variable composition, con- 
taining glutenin, gliadin, globulin, albumin and so on 
in different proportions. The baking properties of 
flour, that is, its ability to hold water and the leaven- 
ing bubbles of carbonic acid gas evolved by the yeast, 
depend mainly on the physical condition of its gluten. 
This in turn depends on the relative ratio of the several 
colloids, and especially on the presence of salts which 
materially affect their solubility and their protective 
relation to each other, as is evident from the fol- 
lowing: 

Gliadin forms an opalescent colloidal solution in 
distilled water, from which it is precipitated by sodium 
chloride. 

Glutenin is insoluble in water or saline solutions. 
It dissolves in dilute acids (or alkalis), and is thrown 
out again upon neutralization. 

Globulin and albumin are soluble in sodium chloride 
solutions. 


PRACTICAL APPLICATIONS ieel 


The various gluten fractions can hardly be said to 
be definite chemical individuals, and their purifica- 
tion, followed by an investigation of their mutual 
influence on each other, and also the influence of salts, 
constitutes a fruitful field for research. It is prob- 
able that we shall find here an instance of cumulative 
protection, where, for example, the protective action 
of the gliadin on the glutenin is enhanced by the pro- 
tective action of the globulin and albumin on the 
gliadin. Salts, by changing the degree of dispersion 
or solubility of the several fractions, must shift the 
equilibrium of the complex system. So also does the 
slight acidity developed by the yeast. Thus, Weyland 
Bischoff (Jago, ‘‘The Technology of Bread-making’’) 
found that flour moistened with a 15 per cent. solu- 
tion of sodium chloride yielded a dough deficient in 
tenacity. Flour heated for several hours at 60 deg. C. 
will not make a dough, probably because one (per- 
haps more) of its colloids has lost so much water that 
the constituent particles have approached too close 
to be separated by water alone. This is similar to the 
action of gelatin, which loses its solubility upon being 
heated at 110 deg. C. or above. 

The best results with flours are obtained by using 
water containing small amounts of sodium chloride. 
Salt, used from time immemorial, besides adding to 
palatability, enables the gluten to absorb and hold 
water, an effect shared by chlorides generally. Hard 
waters, containing sulphates or the like, harden the 
gluten; whereas, soft alkaline waters disintegrate it 
and destroy its elasticity. Even distilled water yields 
a sticky dough. 

The importance of water in bread-making is obvi- 


128 COLLOID CHEMISTRY 


ously on a par with its importance in brewing, and in 
both cases the mineral impurities affect the yeast. 
Alum, calcium sulphate and even sulphate of copper 
have been used to improve gluten, especially in spoiled 
flours. Potassium bromate, recently introduced as a 
‘“‘yeast food,” also registers its effect on the colloidal 
condition of the gluten. 

While the carbon dioxide formed by the yeast is 
initially liberated in nearly colloidal dispersion, it 
soon forms visible bubbles, which, however, must be 
kept in fine dispersion by the elastic gluten if a loaf 
of good texture is expected. The addition of a little 
fat to the dough and the use of, say, half milk and 
half water, both serve not only to improve the texture 
of the loaf, but to add materially to its keeping proper- 
ties. Whymper attributes the keeping properties of 
the home-made farm loaf to the protective action 
of fats on the starch—that is, fat tends to prevent staling 
of bread. The casein and salts of the milk also 
affect the gluten, and the milk fat or other added fat, 
by going into emulsion, tends to coat over the dough 
particles and prevent desiccation. Such action of the 
fat results in shortening, the resulting bread, cake or 
biscuit being flakier and more brittle. 


Milk 
From a colloid chemical standpoint, the main con- 
stituents of milk may be classified as follows: 


In crystalloid dispersion: 
Salts (e.g., NaCl). 
Sugar (lactose). 


PRACTICAL APPLICATIONS 129 


In colloidal dispersion: 


Casein, an unstable or irreversible colloid. 
Lactalbumin, a stable, reversible, protective col- 
loid. 


In suspension or emulsion: Milk fat.* 


Most formulas and recipes for modifying cows’ milk 
for infant feeding and almost all analyses of milk com- 
bine the percentages of casein and lactalbumin under 
the collective title of ‘‘total proteids’”’ (in modern 
form, ‘‘total protein’; globulin is also included), 
thereby obscuring the highly important fact that the 
lactalbumin stabilizes and protects the casein from 
coagulation by acid and rennin. 

The subjoined table, giving average compositions 
in per cent., shows how natural milks are influenced 
by variations in the casein/lactalbumin ratio: 


Kind Lact- Pro- Behavior with 
of Casein. al- tective | Fat. acid and 
milk. bumin. Ratio. rennin. 
COs «css 3.02 0.53 0.14 3.64 | Readily coagulated into 


large curds. 
Woman .| 1.03 1.26 1.13 3.78 | Not readily coagulated; 


forms small curds. 
LA ae 0.67 1.55 2.31 1.64 


It is a striking fact that in the above table the milks 
are arranged in order of their digestibility by infants, 
which corresponds with their relative degree of col- 
loidal protection. Thus, A. Jacobi [‘‘The Intestinal 
Diseases of Infancy and Early Childhood,” N. Y., 
1889] stated that asses’ milk has been recognized as a 
refuge in digestive disorders in which neither mothers’ 
milk nor cows’ milk nor mixtures were tolerated. 

* Tt is probable that of some the milk fat is in colloidal dispersion. 


130 COLLOID CHEMISTRY 


The addition of protective colloids to cows’ milk 
stabilizes it and makes it behave like mothers’ milk, 
when treated with acid or rennin. In modifying 
cows’ milk for infant feeding, the usual dilution, fol- 
lowed by an adjustment of the fat and lactose ratios, 
is, of course, necessary. If enough protective colloid 
be added, coagulation of casein in the stomach may be 
entirely prevented, or, at least, the coagula kept in a 
very fine state of subdivision. 

Besides stabilizing the casein, protective colloids 
have a very important effect on the milk fat, on which 
they exercise an emulsifying and emulsostatic in- 
fluence. Indirectly their stabilization of the casein is 
of much greater importance, because insufficiently 
protected casein in curding, entraps mechanically most 
of the milk fat, making a fatty, greasy curd which 
tends to repel the acid gastric juice and pass undis- 
solved or only partially dissolved into the intestine, 
where its putrefaction causes trouble. 

It is probable that the relative percentage of 
lactalbumin decreases with the period of lactation, 
so that the milk of a herd is in this respect more uni- 
form than the milk of a single cow. With nervous 
women, nervous shock probably decreases the amount 
of albumin secreted, and this would register itself in a 
digestive upset in the nursling. The protective ratio 
of any particular mother’s milk is an important factor 
in its usefulness to the infant. 


Although their action was not perfectly understood, 


the most diverse kinds of protective colloids have for 
years been used in the modification of cows’ milk for 
infants. For over thirty years Jacobi advocated the 
use of gelatin and gum arabic, and the use of cereal 


- 


PRACTICAL APPLICATIONS od 


gruels and dextrinized starch is familiar to all. Beer, 
seaweed (Irish moss) and lichens (Iceland moss) are 
among the protective colloids used in other lands, and 
in England especially, sodium citrate is much used, 
This protective salt, when going into solution, exhibits 
in the ultramicroscope actively moving ultramicrons, 
a fact indicative of its colloidal condition. 

The action of protective colloids is beautifully illus- 
trated in the ultramicroscope, which enables us to see, 
in very highly diluted milk, the individual colloidal 
particles of casein in active motion, and to watch the 
course of their coagulation by acid or rennin. The 
casein ultramicrons of cows’ milk first form groups of 
two or three, whose motion is somewhat hampered, 
and they continue to aggregate into larger and still 
larger groups whose motion decreases as their size in- 
creases, until they finally sink out of solution in coagu- 
lated masses. If, however, some gelatin or gum arabic 
solution be added to the diluted milk before the addi- 
tion of the acid or rennin, the casein particles continue 
their active ‘‘dance” and do not coagulate. In this 
connection it is interesting to note that the casein 
particles of mothers’ milk appear to be much smaller 
than those of cows’ milk, probably because of the more 
highly protective medium in which they are formed and 
exist. [See J. Alexander, Kolloid-Zeit. 1909, 1910; 
J. Am. Chem. Soc. 1910, 28, 280; Alexander and Bul- 
lowa, Arch. Pediat. 1910; J. Am. Med. Assoc. 1910.] 

Since E. Zunz showed that certain albumoses 
exerted a coagulative rather than a protective action, 
J. Alexander digested lactalbumin with rennin or 
rennet, and found that the resulting digestion products 
no longer acted as protectors in forming and stabilizing 


182 COLLOID CHEMISTRY 


colloidal precipitates (for example, AgCl), but actually 
tended to act as coagulators. He therefore advanced 
the following simple colloid-chemical explanation of 
the rennin coagulation of milk [8th Int. Cong. Appl. 
Chem. 6, 12-14 (1912)]: (1) The enzyme rennin 
digests the lactalbumin, thereby destroying its pro- 
tective action; (2) the resulting albumoses are prob- 
ably coagulators; (3) calcium salts liberated from their 
adsorption by the lactalbumin and the like also aid in 
the coagulation. 

When cream, or milk, heated to 50 to 60 deg. C. is 
forced through tiny orifices under great pressures (the 
pressures used are 200 atmospheres and more, and 
the fluid is usually spattered against an agate plate), © 
the fat globules are much more finely dispersed, being 
reduced to about one hundredth their former diameter. 
As aconsequence of the enormous increase in the free or 
specific surface of the system, the viscosity of such 
homogenized milk is greatly increased. The finely di- 
vided fat adsorbs so much lactalbumin (Wiegner esti- 
mates that it adsorbs about 25 per cent., as against 
about 2 percent. in ordinary milk), and is so highly 
stabilized thereby, that homogenized milk will not yield 
butter on churning, nor does homogenized cream churn 
or whip. On the other hand, because of its viscosity, 
homogenized cream with 17 per cent. butter fat will 
make as well-bodied an ice cream as an ordinary 25 per 
cent. cream. In fact cream is now largely kept in cold 
storage in the form of sweet butter, which is homo- 
genized with sweet milk in the busy season, yielding 
cream again. 

In whipped cream coagulation of the lactalbumin 
may be a factor too, and with Charlotte Russe and 
Bavarian creams gelatin is added. 


PRACTICAL APPLICATIONS 133 


Artificial milks are made by emulsifying vegetable 
fats or oils with protective colloids or with skim milk. 
Most of them are deficient in fat-soluble vitamine A. 


Ice Cream 


One of the essentials in ice cream is that when it is 
served it shall have a smooth, mellow, velvety texture, 
and it is a fact amply proved by the experience of 
practical ice cream manufacturers and housewives, 
and backed by the authority of even very old cook 
books and recipes, that ice cream made without the 
addition of eggs, gelatin, or some similar protective 
colloid, is gritty, grainy, or sandy, or else soon be- 
comes so on standing. The original standard for ice 
cream, promulgated under the Foodand Drugs act, took 
no note of these facts, and fell when they were brought 
out in court during an attempt to justify this standard. 

The added protective colloid tends to inhibit the 
erystallization of the water with the formation of 
sharp spicules of ice. Furthermore, it also stabilizes 
the casein, a matter of the highest importance; for 
as ice cream always contains considerably more fat 
than milk, the curds formed by unprotected casein 
would be particularly greasy and hard to digest. 

A very misleading impression has been given by 
some food officials referring to gelatin in ice cream as 
a “filler,” which naturally leads to the idea that it is 
an inferior ingredient added in quantity to cheapen 
the product. But as gelatin is expensive and as only 
about 0.5 per cent. is used, such a view is erroneous. 
The food value of gelatin as a protector of the body’s 
nitrogen being generally admitted, and its effect being 
very beneficial from a digestive point of view, its 
use in ice cream is necessary, legitimate and scientific. 

10 


134 COLLOID CHEMISTRY 


Confectionery 


Originally ‘‘gum drops” were made with gum arabic 
as a protective colloid to prevent the crystallization of 
the sugar and give the candy a smooth agreeable body. 
For cheaper grades glucose and soluble starches are 
now used; for, although the soluble starch is not a 
powerful protector, it is inexpensive, considerable can 
be used, and commercial glucose has but slight 
tendency to crystallize as about half of its dry weight 
consists of a highly dispersed and protective dextrin. 
Glucose containing too much dextrose crystallizes on 
standing. 

‘‘Marshmallows” are usually made with gelatin as 
the emulsifying, body-forming, protective colloid, but 
albumen and gum are also used. 

In molding candies a super-dried starch is used 
containing only a few per cent. of moisture. Its 
powerful attraction for moisture tends to keep the 
candy in shape. Our ordinary fruit jellies and jams 
contain protective colloids such as pectin, which not 
only give body, but usually prevent the crystallization 
of the large quantity of sugar present. 


Gelatin and Glue 


When bones, hide, or skins are heated with water, 
especially after a preliminary treatment in lime water 
followed by thorough washing, there is formed a 
solution that gelatinizes when cold. If the raw 
material is carefully selected and treated, it yields 
when dried a light colored transparent gelatin; if 
made without the precautions that should surround 
a food product, the result is glue. 

Since the gelatins from different sources vary con- 


PRACTICAL APPLICATIONS 135 


siderably in the percentages of amino-acids they yield 
on hydrolysis, it seems obvious that gelatin is not a 
definite chemical entity, but is rather an adsorption 
complex whose structure is at least duplex and is 
probably even more complicated. The various con- 
stituents of the gelatin bear a cumulative protective 
relation to each other (see p. 46). 

Gelatin exhibits its minimum degree of swelling at 
its iso-electric point, about pH 4.7, and a maximum 
of swelling at about pH 3, after which more acid 
causes contraction again. J. Loeb has explained this 
and many other phenomena exhibited by gelatin on 
the basis of the Donnan theory of membrane equi- 
librium, assuming that the gelatin forms salts having 
a non-diffusible ion (see ‘‘ Proteins and the Theory of 
Colloidal Behavior,’’ 1922). The facts are just as 
well explainable on the basis of a kinetically balanced 
adsorption, and in view of the variable composition 
of gelatin, it seems idle to speak of definite salts like 
“gelatin chloride’ or ‘‘sodium gelatinate.’”’ The 
presence of small amounts of adsorbed impurities, 
tenaciously held, exercises a marked effect. For this 
reason the so-called ‘‘water absorption test’’ for 
gelatin and glue is uncertain. 

Generally speaking, the binding strength of a 
gelatin or glue is measured by its relative jelly strength 
and the viscosity shown by its solution; but the varia- 
tions in these two factors do not always parallel each 
other. Heating causes a breakdown of the gelatin 
into smaller complexes; this lowers viscosity, Jelly 
strength, and binding power, indicative of a drop 
from the zone of optimum colloidality. On the other 
hand, as glue solutions cool and the colloid begins to 


136 COLLOID CHEMISTRY 


form large aggregates, the adhesiveness also falls off 
(see p. 17). In choosing a glue or a gelatin for any 
specific purpose, all of its qualities and attributes 
must be taken into consideration, for foam, grease, 
color, odor, etc., may be of crucial importance (for 
further details see Am. Chem. Soc. Monograph ‘‘ Glue 
and Gelatin,” by J. Alexander). 


CHAPTER 13 
PRACTICAL APPLICATIONS (Continued) 


GLASSES 

While glasses consist of mixtures of silicates con- 
taining dissolved or finely dispersed impurities or addi- 
tions, and while individual silicates may be isolated 
from glass, the constitution of the glass as a whole is 
not quite so simple as might be assumed from paper 
formulas. For it is the physical properties of glass 
that are the most important, and while these naturally 
change as the ingredients vary in kind and proportion, 
still the properties of parts of one and the same batch 
may differ widely if they are subjected to different 
heat treatment. Thus Prince Rupert drops, made by 
dropping part of a fluid batch of opal glass into water 
so that the surface was quickly chilled from 1200 deg. 
C. down to 20 deg. C., were clear at the exterior, be- 
coming progressively more opalescent toward the 
center. Slowly cooled drops of the same batch were 
quite opaque throughout. 

The composition of glass is so chosen that the several 
silicates mutually interfere with each other’s crystalliza- 
tion (cumulative protection), the result being a colloidal 
mass in which the tendency toward crystallization may 
register itself by devitrification, a formation of relatively 
largecrystals which render the glass turbid and injure its 
usual working properties. While many silicates may 
be cooled quite slowly without crystallizing, the sili- 
cates of calcium, magnesium, and aluminum form 
crystals if not quickly chilled, which is rather difficult 
to do because silicates in general have high thermal 
capacity but low radiating capacity. 

137 


138 COLLOID CHEMISTRY 


Tammann (Zeit. Elektrochem. 1904, 10, 502) has 
pointed out three main factors controlling the behavior 
of supercooled melts: (1) The specific crystallization 
capacity (measured by the number of crystallization 
centers formed in per unit mass per unit time); 
(2) the speed of crystallization; (3) the variation in 
viscosity. ‘To these must be added a fourth; mutual 
or cumulative protection. 

Every substance in forming macroscopic crystals 
must of necessity pass through the colloidal zone, in 
which surface forces (adsorption, surface tension) 
exercise a controlling, if transient, influence that may 
be made permanent if sudden cooling increases the 
viscosity sufficiently. The adsorption of the various 
‘silicates by each other as they reach colloidal dimen- 
sions introduces a time-lag which becomes cumulative 
as the several silicates protect each other, and greater 
as the viscosity increases. Batches of glass are so 
mixed as to make this time-lag ample to prevent 
crystallization at the temperatures and during the 
times of melting, casting, molding and so on. When 
glass is cooled from the plastic to the rigid state, as 
in blowing, drawing, casting, etc., and especially 
when optical glass is being slowly cooled in melting 
pots, devitrification may occur through the separa- 
tion of spherulites or minute crystals finely dispersed 
throughout the glass. The spherulitic form is usually 
indicative of crystallization in the presence of a col- 
loid, and the fact that the crystals are by microscopic 
examination usually found to be tridymite and erysto- 
balite points to the probability that some of the silica 
dissolves colloidally in the molten silicates. Sul- 
phates, chlorides, an excess of arsenic, and sometimes 


PRACTICAL APPLICATIONS 139 


fluorine, facilitate devitrification, thus exercising a 
‘‘salting-out”’ action. 

Practically all transparent colored glasses owe their 
color to the presence of colloidally dispersed sub- 
stances, usually metals or their oxides. The gold 
ruby glasses were carefully investigated by Zsig- 
mondy [‘‘Colloids and the Ultramicroscope”’ (trans. 
by J. Alexander), J. Wiley & Sons Co.]. When gold 
is added to a batch of glass (usually 0.03 to 0.01 per 
cent. of gold chloride), the melt is colorless and may 
even remain so on slow cooling unless the batch is 
regulated to prevent this. All batches, however, if 
quickly chilled, are colorless and optically clear in the 
ultramicroscope. ‘The color is developed by reheat- 
ing the colorless glass to the softening point, whereby 
the dissolved or crystalloidally dispersed gold begins 
to separate out upon nuclei (‘‘crystallogens’’) already 
formed in the colorless glass. 

If the composition and heat treatment of the batch 
has been such as to yield a large number of small 
nuclei, the color developed is a bright ruby red; for 
although each of these amicroscopic particles grows 
to colloidal dimensions, the available gold is used up 
before any of them become big enough to render the 
glass turbid. On the other hand, if a small number of 
larger nuclei be present in the colorless ruby glass, 
- development leads to a dirty blue or turbid maroon 
shade because the dissolved gold is deposited in fewer 
and larger masses. 

The subjoined table shows the results of Zsig- 
mondy’s ultramicroscopic examination of good and 
of spoiled ruby glass. Both specimens had been 

cooled slowly and reheated more rapidly until one 


140 COLLOID CHEMISTRY 


began to melt (hot end) while the other end remained 
cold (cold end). The fact that the average distance 
between ultramicrons remained constant in all parts 
of the specimens proves that reheating formed no 
new nuclei. 


Hot End Goop Rusy Guass SPoILED Rusy Guass 
Color intense red. Nu- Color blue. Ultrami- 
merous green ultrami- crons fewer, copper-red, 





S crons, very close to- | ¢ further apart. 

§ gether, their brightness So 

ty diminishing. { g ; b 

By ty Color violet. Ultrami- 

o @ crons yellow. 

$ 3 

m Dn 

® Homogeneous green ® Color bright red. Ultra- 

EB light cone. | microns green. 

' Colorless and homogene- Colorless. Few faint 
ous. specks visible. 


Cold End 


H. F. Bellamy’s results (J. Am. Ceramic Soc. 1920) 
show that the stannic oxide used in gold ruby glass 
acts as a protector to the gold, keeping the color true 
and deepening it so that less gold can be used. This 
recalls the work of Zsigmondy, who, by mixing the 
hydrosols of gold and stannic acid, synthesized the 
purple of Cassius (an ammonia-soluble deep red pre- 
cipitate obtained by mixing solutions of auric chloride 
and stannous chloride), which Berzelius had regarded 
as a chemical combination of the tin sesquioxide with 
purplish oxide of gold. 


Metals and Alloys 


Coarsely crystalline metals are brittle, because they 
tend to split along the lines of crystal cleavage, and 


PRACTICAL APPLICATIONS 141 


therefore the metallurgist usually aims to produce a 
fine-grained structure. Among the physical methods 
used to achieve this end are chilling and rolling, while 
the chemical methods involve the removal of unde- 
sirable constituents (as in the conversion of pig iron 
into steel), and the addition of desirable constituents 
(carbon in case hardening, various metals in alloy 
steels). Thus Pitz found that the predominant 
effect of vanadium in steel is to decrease the size of 
the ferrite grains, and make the pearlitic structure 
fine-grained and homogeneous, yielding a harder steel. 

The chief cause of fine-grained structure seems to 
be the inhibition of crystallization by substances col- 
loidally dispersed in the metal. These may be other 
metals, such as chromium, nickel or tungsten (or their 
compounds) in alloy steels, compounds such as Fe;C 
in ordinary steel, or even the metal itself in the case 
of pure metals. The last-mentioned phenomenon is 
consequent on 7tso-colloidism (a colloidal dispersion of 
the metal in itself as dispersing phase), which may 
lead to auto-protection, the colloidal phase tending to 
interfere with the crystallization of the rest. Several 
organic compounds exhibit a similar phenomenon, 
yielding amorphous gels which gradually become 
visibly crystalline (seep. 45). It must be remembered 
that size alone is the criterion of the colloidal condition. 
As Scherrer has shown with the X-ray spectrometer, the 
ultramicrons and even the amicrons in colloidal gold 
hydrosols consist of tiny crystalline groups; colloidal 
silicic and stannic acids showed both crystalline and 
random or haphazard molecular clusters, while gelatin 
was entirely amorphous. 

These experiments give us an inkling as to what 


142 COLLOID CHEMISTRY 


occurs during the ‘‘heat treatment’’ and tempering of 
metals, and it is to be hoped that some technique may 
be devised that will give us even a clearer insight than 
does ‘‘etching”’ into the changes that occur in metals 
in metallurgical operations (heat treatment), use, age, 
and even “‘disease”’ of tin for example. 

With metals the crystallization forces are so power- 
ful and act so rapidly, that a coarsely crystalline 
structure usually results on ordinary slow cooling. 
To preserve the fine colloidal structure, even in the 
presence of protectors, drastic chilling or quent es is 
often necessary. 

The Time Factor 


A fact of general importance in nature, having 
especial application to the phenomena met with in 
metals and alloys, is that many transformations occur 
so rapidly as to elude our observation, and compel us 
to draw upon our imagination to follow what happens 
between the initial point and the end-point.* By 
photographs taken thousandths of a second apart, 
Rayleigh showed the curious differences that occur 
when various liquids are dropped into water. 


Iron and Steel 


When treated with dilute acid, drastically quenched 
tool steel does not separate out carbide of iron, but 
yields instead a complicated mixture of hydrocarbons. 
This shows that the Fe3C is in such a fine state of sub- 
division that upon its decomposition the nascent hy- 

* T. W. Richards [Am. Chem. J., 26, 61 (1901)] followed microscop- 
ically by instantaneous photography, the separation of crystals from 
solutions, and reported that the initial growth is much more rapid than 
subsequent growth. V. Henri applied the ‘moving picture” camera 
(cinematograph) to the ultramicroscope, and E. O. Kraemer, National 
Research Fellow at the University of Wisconsin, is making remarkable 
films of ultramicroscopic happenings. 


PRACTICAL APPLICATIONS 143 


drogen and carbon are within the range of molecular 
attraction which is of the order of 50 millimicrons; 
and the Fe3C is in colloidal state. 

Such drastically quenched steel owes its great hard- 
ness to its enormous free or specific surface, but it is 
too brittle to be of practical use, and must therefore 
have its hardness drawn or reduced. This tempering 
process, as it is called, consists in reheating the steel 
and keeping it at certain temperatures for various 
times. Assoonasthe temperature of the steel permits, 
the aggregation of the Fe;C (or cementite, as it is 
known), which was suspended by the previous chilling, 
begins anew. Metallurgists recognize the following 
forms of the iron-iron carbide dispersion: 


Tron-Cementite Nature of 
Dispersions. Dispersion. Crystal Structures. 
Austenite...... KF “Solid solution”? . .Structureless streaks. 
Hardenite...... 3 OMOIGS Lg. a5 cle <> Structureless martensite. 
mM 
Martensite..... Ez COHOICGL ons a a’ « Generally cee but varies; 
‘ 3 may be very 
Troostite...... &, Coagulation begun . Rounded or adie (globulitiey: 
@) 
Sorbite........ 3 ReOGCULUTS oo. ss Fine pearlite, not microscop- 
os ically resolvable. 
Pearlite....... s’ Coagulum......... Laminated; may segregate 
into balls. 


Hardenite is named from its intense hardness; pearlite from its 
pearly luster; the rest are named after the metallurgists Roberts- 
Austen, Martens, Troost and Sorby, respectively. 

The Fe3C in austenite is partly, if not entirely col- 
loidal. Hardenite and martensite represent the hardest 
dispersions, while troostite, sorbite and pearlite repre- 
sent a decreasing order of hardness. We have here a 
typical instance of a zone of maximum colloidality, such 
asis also met within the alloy duralumin. (See J. Alex- 
ander, J. Am. Chem. Soc. 43, 434 [1921]; Mercia, Walt- 
enberg and Scott, Bull. 150, Bureau of Standards, 1919; 


144. COLLOID CHEMISTRY 


Jeffries and Archer [Chem. Met. Eng. 24, 1057, 1921] 
call it a “‘critical dispersion.”) Mercia and Jeffries 
and Archer ascribe the hardening of duralumin to the 
precipitation of colloidal CuAk, whereas the British 
National Physical Laboratory believe that Mg.Si is 
responsible. Probably both compounds are involved, 
and we may have here another instance of cumulative 
protection, where a protector is itself protected. 

Jeffries and Archer* believe that this phenomenon in 
steel, an analogue of which occurs in the alloy duralu- 
min, is due to the fact that the hard particles of the dis- 
persed phase act as keys, preventing any motion along 
the cleavage or slip planes as a whole. While this 
mechanical comparison may appeal to engineers, the 
action of surface forces, together with the fineness of 
crystal grain which minimizes the length of the slip 
planes, is quite sufficient to account for the phe- 
nomena observed. 

The enormous power of these surface forces is shown 
by the results of some experiments privately com- 
municated to me by P. Scherrer of Zurich. He co- 
agulated unprotected gold sols with electrolytes, and 
by X-ray examination found that the tiny ultra- 
microns (which were about 2 yy in diameter and 
showed a crystalline gold space lattice) practically 
fused together into homogeneous crystals of larger 
size. Even soft substances, if finely dispersed, will 
produce great increase in hardness. Thus a few per 
cent. of oil added to whiting makes a firm putty when 
ground in with it. And 8. U. Pickering + made an 
extremely stiff mass by emulsifying 99 per cent. of 
petroleum oil in 1 per cent. of soap water. 


* Chem. Met. Eng., 24, 1065 (1922). 
T J. Chem. Soc., 91, 2002 (1902). 


PRACTICAL APPLICATIONS 145 


The size, shape, initial heating and chilling of the 
piece of steel, as well as the speed, temperature and 
time of its reheating or tempering and speed of its 
second chilling, are all factors controlling the nature 
of the final dispersion, which is also largely influenced 
by the chemical composition of steel itself (percentage 
of the Fe;C, presence of other metals and of impurities). 

As a consequence, the proper tempering of steel 
requires much experience in the practical control of 
conditions, otherwise the optimum point is either 
passed or not reached; and for most uses the optimum 
dispersion of the cementite lies within or just beyond 
the colloidal zone. The addition of manganese (m. 
p. 1225 deg. C.) to steel retards the aggregation of the 
cementite to such a degree that ordinary slow cooling 
yields martensite. Steels containing tungsten and 
other metals of high melting point are known as 
“‘high speed”’ steels because lathe tools made with 
this highly protected steel maintain their hardness 
even though brought to a low redness by the high 
speed of the lathe. Undersuch circumstances ordinary 
tool steel at once loses its temper, but the high speed 
steels are ‘‘self-tempering.”’ 

The behavior of the iron-carbon alloys is greatly 
influenced by the allotropic changes of iron. 

After ‘“‘freezing”’ at about 1505 deg. C., pure iron 
shows upon further cooling a large evolution of heat at 
about 900 deg. C. known as thermal arrest point 
(Ar3), and a smaller evolution of heat at about 780 
deg. (Ar). Above 900 deg. the iron exists in the 
non-magnetic or gamma form,* and below 780 deg. 


* A new form, delta iron, is claimed by Honda to exist at higher 
temperatures. 


146 COLLOID CHEMISTRY 


in the magnetic or alpha form, which exhibits a dif- 
ferent crystallization from gamma iron. Between 
these temperatures, Rosenhain believes that the iron 
exists in a third allotropic form, known as beta iron, 
which has the crystal form of alpha iron but is non- 
magnetic like gamma iron. Benedicks, on the other 
hand, believes that the evolution of heat at 780 deg. 
does not indicate the change of a beta allotrope into 
gamma iron, but represents the final disappearance of 
persistent gamma iron molecular groups from the 
metal. 

These apparently conflicting views can be recon- 
ciled by regarding the iron between 900 deg. and 780 
deg. C. as an allocolloid or allodispersoid, the so- 
called “‘beta”’ iron being an alpha-gamma, adsorption 
compound (alpha iron dispersed in gamma, iron), 
whose breaking up into the more completely orientated 
alpha iron sets free the relatively smaller amount of 
heat at the point Ar,. This view accounts for the fol- 
lowing facts: (1) Betairon has the same crystal form as 
its “predominant partner,” alpha iron, but is not mag- 
netic;* (2) gamma iron dissolves carbide, whereas 
beta iron and alpha iron do not; whatever gamma 
iron exists in the beta zone is adsorbed by or in some 
loose kind of combination with the dominant alpha 
iron, and is not free to exert its solvent action on 
iron carbide; (3) the increase in strength when alpha 
iron is transformed into beta iron; as the temperature 

* Magnetism seems to depend not on chemical composition, but 
rather on a peculiar regular molecular orientation. The so-called 
“Haeussler alloys” are magnetic, although their individual constituents 
are not. According to A. W. Hull [Phys. Rev., 14, 540 (1919)], mag- 


netism depends on the distance between atoms rather than their 
arrangement. 


PRACTICAL APPLICATIONS 147 


goes over 780 deg. C., some gamma iron forms, and 
the resulting alpha-gamma adsorption compound 
increases the total free surface and strength of the 
system; (4) the fine acicular structure of beta iron; 
this is indicative of crystallization in the presence of a 
colloid. 

Steel 

The preceding remarks apply to pure iron. With 
the introduction of carbon, we have to deal with the 
iron carbide FesC (cementite), and the system is com- 
plicated accordingly, dislocating the thermal arrest 
points. 

A low-carbon steel (containing, say, 0.2 per cent. 
of carbon which dissolves in the liquid iron as Fe;C) 
when freezing down to 840 deg. C. consists of an 
apparently homogeneous solid solution or dispersion 
of cementite in gamma iron. At 840 deg. C. the 
alpha-gamma, dispersion (beta iron) forms (Ar;), and 
at 750 deg. C. the residual alpha-gamma adsorption 
compound decomposes into alpha iron (Ar). The 
steel now consists of alpha iron crystals dispersed in a 
‘‘solid solution”’ of iron carbide in gammairon. With 
further cooling the quantity of alpha iron crystals 
increases until at a little below 700 deg. C. the re- 
maining gamma iron-cementite solid solution is trans- 
formed into a eutectoid mixture of alpha iron (ferrite) 
and iron carbide (cementite) with an evolution of 
heat (Ar). 

This last transformation seems to be the breaking 
up of an adsorption compound between gamma, iron 
and iron carbide, the former transforming into alpha 
iron and the latter being set free to form aggregations 
of its own. This adsorption compound carries over 


148 COLLOID CHEMISTRY 


some of the gamma iron through the so-called ‘‘ beta” 
zone to the eutectoid point, where, with increasing 
carbon content, increasing quantities of heat are 
evolved. The allotropic transformation of the iron 
becomes practically negligible when the steel contains 
above about 0.5 per cent. of carbon (which, however, 
means about 7.5 per cent. Fe;C). Steel of eutectoid 
composition (about 0.9 per cent. Fe, equaling about 
13.5 per cent. Fe;C) exhibits only a single thermal 
arrest point, at which it glows visibly. This phe- 
nomenon, known as realcalescence, indicates a sudden 
large release of energy consequent to the breakdown 
of the extensive metastable system gamma iron- 
cementite, whereby two delayed transformations oc- 
cur simultaneously—the iron transforms from gamma 
into alpha iron (ferrite), and at the same time the 
released cementite aggregates. The inhibition of the 
allotropic iron transformation shows that in eutectoid 
steel the large percentage of Fe;C has carried over 
practically all the gamma iron in metastable condition 
to the eutectoid point. The presence of such ele- 
ments as silicon and manganese perceptibly changes 
the location of the eutectoid point, illustrative of the 
easy disturbance of colloidal systems by foreign sub- 
stances. 

The iron carbide (cementite) possesses considerable 
cohesion and attempts to aggregate against the re- 
sistance offered by the now highly viscous iron—an 
ideal condition for the development of a colloidal 
system. ‘The cohesive power of the cementite is so 
great, however, that in unquenched steels it usually 
appears interspersed with alpha iron (ferrite), as 
fine plates or fibrils, yielding the finely laminated 


PRACTICAL APPLICATIONS 149 


structure known as pearlite because under proper 
illumination it exhibits the iridescence of mother of 





x 500 
Fig. 2. Microstructure oF 0.85% C. Srern, HEATED To 800° 
AND COOLED AT DIFFERENT RATES. ETcHED IN 5% PICRIC ACID IN 


ALCOHOL. 

Note how the lamellae constituting the pearlite become much thicker 
and more pronounced as the annealing progressed. The physical 
properties of the material also changed in corresponding manner. 

(a) cooled in air—largely sorbite. 

(b) cooled in lime—fine lamellar pearlite and some sorbite. 

(c) cooled in furnace—coarse lamellar pearlite showing some sphero- 
dizing. : 

(d) cooled in furnace at much slower rate than (c); the pearlite is 
largely spherodized or “‘divorced.”’ 

(from U. S. Bureau of Standards, Circular 113 (1922)) 


150 COLLOID CHEMISTRY 


pearl. The iridescence is evidence of the fineness of 
the pearlitic structure, which, like the diatom Pleuro- 
sigma, taxes the highest powers of the microscope for 
its resolution, the lamelle being often less than 0.2 
micron apart. 





x 500 
Fig. 3. VERY FINE MARTENSITE IN 0.46 CARBON STEEL, quenched 
in water after heating 15 minutes at 850° (1515° F.), just above the A» 
transformation. Etched with 2% alcoholic solution of nitric acid. 


Microscopically, eutectoid steel (about 0.9 per cent. 
C.) when slowly cooled, consists entirely of pearlite, 
thus corresponding to the pure eutectic of ordinary 
alloys.* But with very slowly cooled steel, or with 
steel reheated for a long time at about 900 deg., the 
cementite tends to form balls or globules, the liber- 
ated ferrite at the same time forming larger crystals. 

The following table shows the inter-relation of these 
phenomena: 


* As the Greek-derived word indicates, a eutectic is the most easily 
fusible mixture of two metals or other substances. 


151 


PRACTICAL APPLICATIONS 


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152 COLLOID CHEMISTRY 


The following cooling curves taken from Prof. 
H. C. H. Carpenter [Engineering, 107, 341, 1919] 
show how with growing carbon content the three 
thermal arrest points are combined into one in eutec- 
toid steel: 






OO EN |G 
HT Aa taoket pane ieee ae IEICE et awe 
ac apie Uatanqasctes 
aha SRakS Sam Gn RSE REE EH TREE 













Fig. 4.—Changes in thermal arrest points of steels with increasing 
carbon content. 


Stepped Transformation in Steels 


The aggregation of cementite takes time and there 
is always a time lag in the process. Consequently 
when carbon steel is rapidly cooled, the A; transforma- 
tion takes place at a lower temperature than when it is 
slowly cooled. This drop increases with the speed of 
chilling until it reaches about 600° C., when still more 
drastic chilling lowers it discontinuously to about 
300° C. or less. 

This stepped Ai transformation as it is known, is 
beautifully explained by Kétaréd Honda who showed 
that it is due to the fact that in the inhibited zone 
the viscosity of the dispersion medium is sufficient to 
stop the aggregation of the cementite, but that below 
this zone the aggregation forces of the cementite 
become so powerful that they then overcome the 
resistance of the iron, notwithstanding the fact that 
its viscosity has likewise increased by cooling. The 


PRACTICAL APPLICATIONS 153 


following curve, taken from Honda’s paper shows how 
two factors operating at various rates over a tempera- 
ture range, may cause puzzling results, unless their 
effects can be separately analyzed. 





AB = tendency curve; CD = resistance curve; EF = observed 
resultant. 

Fig. 5.—Aggregation-viscosity curves, with resultant showing aggre- 
gation inhibited between about 600-300°. 


AB is the transforming tendency or aggregating 
curve, plotted positively; EF is the resultant, which 
represents the extent to which the tendency is able 
to establish itself. 


Standardized Heat Treatment Terms 


A committee appointed by American steel treaters 
has recommended the following standardization of 
terms regarding the heat treatment of steel. The 
relation of many of these terms to what has been 
stated above is manifest. 

1. Annealing. Heating above the “‘critical temper- 
ature’’ followed by a relatively slow rate of cooling. 

2. Loneal. Heating below the ‘‘critical temper- 
ature”’ followed by any rate of cooling. 


154 COLLOID CHEMISTRY 


3. Normalhzing. Heating above the ‘‘critical tem- 
perature”’ followed by an intermediate rate of cooling. 

Note.—In good practice the heating is considerably 
above the ‘‘critical temperature.” 

4. Spheroidizing. A long-time heating at or about 
the ‘‘critical temperature’’ followed by slow cooling 
throughout the upper part of the cooling range. 

Note.—For the purpose of spheroidizing the ce- 
mentite in high-carbon steels. 

5. Hardening. Heating above the “‘ critical temper- 
ature’’ followed by a relatively rapid rate of cooling. 

6. Tempering. Reheating, after hardening, to some 
temperature below the ‘‘critical temperature,’ fol- 
lowed by any rate of cooling. 

7. Carburizing. Adding carbon with or without 
other hardening elements, such as nitrogen, to wrought 
iron or steel by heating the metal below its melting 
point in contact with carbonaceous material. 

8. Casehardening. Carburizing the surface portion 
of an object and subsequently hardening by suitable 
heat-treatment. 

9. Cyaniding. <A specific application of carburizing 
where the object, or a portion of it, is heated and 
brought into contact with cyanide salt. 

By the term ‘‘critical temperature” is meant that 
temperature which is customarily associated with the 
following phenomena: 

(a) Hardening when quenched. 

(b) Loss of magnetism. 

(c) Absorption of heat. 

(d) Formation of solid solution. 

(e) Pronounced refinement of coarse grain upon 
cooling. 


PRACTICAL APPLICATIONS 155 


Tin-Lead Alloys 

All mixtures of tin (m. p. 232 deg. C.) and lead 
(m. p. 327 deg. C.) melt below 327 deg. C.; the alloy, 
63 per cent. tin and 37 per cent. lead, has the lowest 
melting point. If the metals are mixed in other than 
these eutectic proportions, the excess metal tends to 
crystallize out alone, having the largest possible amount 
of eutectic, which solidifies later. Microscopic ex- 
amination shows that the eutectic is not a chemical 
compound; it is usually laminated and can be brought 
into relatively coarse dispersion by slow cooling. 

But some forces do control the formation of the 
eutectic. As the fused alloy approaches solidification, 
lead, having the higher melting point, begins to form 
molecular groups or tiny crystals (‘‘crystallogens’’). 
This aggregation of lead molecules is opposed by the 
more fluid tin, thus lowering the freezing point of the 
mixture. Diminishing thermal agitation finally allows 
the tin to begin to form groups, and a stage is reached 
where both metals exist largely in colloidal state.* 
In this zone a colloidal adsorption compound is 
formed, the ratio, 63 tin to 37 lead, being consequent 
on their specific forces; any excess of tin or lead is 
free to crystallize independently. In the eutectic, 
the lead by adsorption interferes with the crystalliza- 
tion of the tin to such an extent that, if quickly cooled, 
the eutectic structure is extremely fine. 

As with all colloids, this represents a metastable 
condition, and if the temperature is kept within limits 
which permit molecular orientation without disruptive 


* All pure metals in solidifying pass through an isocolloidal zone, the 
narrowness of which is indicated by the sharp peak or cusp in the in- 
verse-rate curve. See W. Rosenhain, “Introduction to Physical 
Metallurgy, pp. 85-87. D. Van Nostrand Co., N. Y., 1914. 


156 COLLOID CHEMISTRY 


thermal agitation, the eutectic undergoes a gradual 
syneresis analogous to coagulation or demulsification; 
a further separation into its constituent phases. The 
laminated structure, common in eutectics, already 
represents the aggregation of the preéxisting col- 
loidal dispersion, as has been especialy pointed out in 
the discussion of pearlite in steel. 

Solder, used by plumbers and others, is an alloy 
consisting of about equal parts of tin and lead. At 
about 210 deg. C. it begins to extrude crystals of lead 
into the fluid eutectic cement, rendering it pasty and 
easy to ‘‘wipe,’’ spread or mold into the desired shape. 
The introduction of the highly crystalline antimony 
(m. p. 630 deg. C.) into tin-lead alloys results in the 
early formation of crystals of antimony or of the hard 
brittle compound Sn-Sb, with the result that the 
ternary alloy is much harder. 

Type metal is one of these ternary alloys whose 
composition varies with price and service conditions. 
It is usually chilled quickly, which is conducive to 
fine structure and hardness. White bearing metals, 
on the other hand, are usually cast in larger masses 
and chill more slowly, their composition is chosen to 
yield hard wear-resisting crystals of Sn-Sb embedded 
in a colloidal plastic eutectic, which can adapt itself 
to the bearing’s irregularities of shape and pressure. 


Zinc-Copper Alloys (Brass) 


Brasses containing less than 30 per cent. of zinc, 
irrespective of their speed of chilling, always consist 
of a ‘‘solid solution” of zinc in copper, known as 
alpha brass. 

This alpha brass is really an adsorption com- 
pound between copper and zinc, in which the large 


PRACTICAL APPLICATIONS 157 


percentage of difficultly fusible copper (m. p. 1083 
deg. C.) aggregates so rapidly that even in so-called 
“quickly cooled”’ specimens, the zinc (m. p. 419 deg. 
C.) is for the most part too finely dispersed to be 
much adsorbed by the copper. Hence the alloy ex- 
hibits a dendritic structure showing a copper crystal- 
lization as modified by adsorbed zinc, the amount of 
which increases from the center outward as the copper 
aggregates and the mother liquor becomes richer in 
zinc.* 

As Rosenhain observes, the percentage of each 
metal and its state of aggregation are momentarily 
varying during cooling, and because of the wide dis- 
parity between the melting points of the two metals, 
homogeneity is favored either by extremely rapid 
quenching from fusion (which tends to prevent the 
copper from aggregating) or by slow cooling and 
annealing, which favor the more complete dispersion 
and adsorption of the zinc (which is segregated by 
ordinary cooling). 

The natural desire to introduce more of the cheaper 
zinc into brass is limited by the fact that with brasses 
containing 30 to 37 per cent. of zinc there appears a 
new, hard, brittle metastable phase, beta brass. Such 
brasses if quickly cooled are relatively hard and brittle, 
but if cooled slowly the beta phase disappears. With 
more than 37 per cent. of zinc, the beta phase is stable 
at all temperatures down to 470 deg. C. and the slowly 
cooled alloys exhibit the duplex structure found in 
Muntz metal (approximately 40 per cent. zinc). 
Carpenter believes that below 470 deg. C. the beta 
phase decomposes into alpha and gamma phases, and 


* Small pieces of alpha brass drastically quenched from fusion in 
liquid air will probably appear homogeneous in the microscope. 


158 COLLOID CHEMISTRY 


in fact that the beta phase is an almost ultramicro- 
scopic mixture of alpha and gamma brass. (This 
indicates colloidal dispersion, favorable to hardness.) 

Rosenhain [loc. cit., p. 145] says that the gamma 
brass ‘‘is exceedingly hard and brittle and its presence 
in the alloys renders them useless for any purpose 
where strength and toughness are required. This is 
a typical example of a law very widely applicable to 
alloys, viz., that those phases of a binary system which 
contain the two elements in anything like equal pro- 
portions are hard and brittle, only the alloys near the 
ends of a series being as a rule sufficiently strong and 
ductile to be of practical utility. We have already 
seen that the beta phase is harder and more brittle 
than the alpha, so much so that the best brasses, in — 
which strength and ductility are of importance, are 
generally made with a zinc content of approximately 
30 per cent., this being the cheapest alloy which does 
not contain the beta phase.” 

With solutions, as von Weimarn has shown [Kol- 
loid-Zeit. 3, 282 (1908); ibid. 4, 27 (1909)], medium 
concentrations are favorable to the development of 
large crystals, whereas dilute and concentrated solu- 
tions both tend to yield colloidal dispersions; and a 
similar condition appears to exist with mixtures of 
pure metals. At both ends of the alloy series appear 
the larger amounts of the so-called ‘‘amorphous”’ 
phase of the metal, which in most cases is stronger at 
ordinary temperatures than the crystalline phase. 
Highly crystalline metal, deficient in the colloidal 
amorphous phase, is weak because it tends to split 
along the planes of crystal cleavage. Weshould there- 
fore expect weakness where the component metals of 


PRACTICAL APPLICATIONS 159 


a binary alloy are about in equal proportions, for this 
concentration works against the development of col- 
loidal metal and in favor of the larger, more perfect, 
but weaker crystals. 


Bronze 


In their article on ‘‘ Alloys’’* Roberts-Austen and 
Neville ,in speaking of the copper-tin alloys containing 
less than 9 per cent. by weight of tin, say that upon 
quickly chilling small ingots from successively lower 
temperatures beginning just above the melting point, 
we thus learn that these alloys (bronzes) ‘‘although 
chemically uniform when solid, are not so when they 
begin to solidify, but that the liquid deposits crystals 
richer in copper than itself, and therefore that the 
residual liquid becomes richer in tin. Consequently, 
as the final solid is uniform, the crystals formed at 
first must change in composition at a later stage. 
We learn also that solid solutions which exist at high 
temperatures often break up into two materials as 
they cool... .’’ The work of Beilby on plastic flow 
and of Benedicks on quick chilling, give an insight 
into the behavior of metals when stressed, worked and 
hardened. 


Amorphous vs. Colloidal Theory 


The amorphous theory of metals, advocated by 
Rosenhain [loc. cit., p. 249] among others, stresses 
particularly the entire absence of regular orientation 
or crystallization in the so-called ‘‘amorphous”’ phase, 
in which the molecules are supposed to be in the 
random, haphazard, and mainly isolated state as- 
sumed to exist in liquids. I believe, however, that 

* Encyclopedia Britannica, 11th ed., Vol. 1, p. 706. 


160 COLLOID CHEMISTRY 


the amorphous phase consists largely, if not entirely, 
of molecular groups, many of which may be oriented 
in the form of ultramicroscopic crystals or crystal 
fragments; and that its properties are due, not to the 
entire lack of orientation among its particles, but 
mainly to the fact that many or most of its molecular 
groups are of collordal dimensions. 

In metals the molecules are very close together and 
have an extremely powerful cohesion and crystalliza- 
tion tendency, so that it does not seem possible that 
the ‘‘amorphous”’ phase can be composed entirely of 
isolated atoms, molecules, or even entirely of non- 
crystalline groups. With the most drastically chilled 
metal, even allowing for the high viscosity and its 
rapid increase, it does not appear conceivable that no 
tiny ultramicroscopic crystals form, or at least that 
there is no grouping, regular or irregular, of the metal 
molecules. Even with gold hydrosols, where the dis- 
persing water exercises a restraining influence, the 
X-ray spectrometer shows that both ultramicroscopic 
and amicroscopic gold particles are crystalline. 

The facts are in harmony with the conception of 
amorphous metal as an isocolloid—that is, as a dis- 
persion of colloidal crystals, crystal fragments, or non- 
crystalline groups, forming a solid gel-like mass. The 
fineness of its particles and the enormous develop- 
ment of free surface found in all colloids are its critical 
factors.* 


* For further discussion of the colloidal state in metals and alloys 
see J. Alexander [Proc. Am. Inst. Min and Met. Eng., Vol. 63, 1919, 
and Vol. 64, 1920; Chem. Met. Eng. 1922; First Colloid Symposium 
Monograph, (Univ. Wisconsin) 1923]. Many of the changes that 
take place with sulphur are analogous to those that occur in metals. 
By chilling sulphur heated to 400 deg. C. in liquid air, von Weimarn 
[Kolloid-Zeit. 6, 250, 1910] obtained a perfectly transparent and ex- 


PRACTICAL APPLICATIONS 161 


Electro-Deposition of Metals 


The powerful effect of protective colloids on de- 
positing metal may be readily shown by dividing a 
solution of lead acetate between two glasses, to one of 
which is added some glue solution. Upon immersing 
a strip of zinc in each glass, the one without the glue 
gives the usual bright crystalline lead ‘‘tree,’’ whereas 
the other gives a very much inhibited or amorphous 
looking deposit. A tin ‘‘tree’” shows similar results, 
but if some lead acetate be added to the stannous 
chloride solution, the lead and tin mutually interfere 
with each other’s crystallization. In refining metals 
electrolytically, new very pure baths usually give 
coarse-grained brittle anodes, but this effect wears 
off as ‘‘impurities”’ accumulate or are intentionally 
added. 

In other cases impurities even in minute percentage, 
are highly injurious. Thus one part of iron in five 
million may injuriously affect the quality of electro- 
deposited nickel. In the electro-deposition of zinc, 
antimony is especially troublesome, even in traces. 

A large variety of addition agents are used, the 
essential being that the added substance must be 
adsorbed by the depositing metal when the latter is 
in the colloidal zone through which it must necessarily 
pass. Miller and Bahntje [Zeit. Elektrochem. 1906, 
12, 317] found that copper deposited in the presence 
of colloids to keep it fine grained weighed 0.2 per 
cent. more than metal deposited without the colloid. 
They found that gelatin had the most powerful effect, 


ceedingly elastic sulphur. Here the rapid cooling results in the tem- 
porary preservation of a relatively large percentage of the viscous Sy 
dispersed in the more fluid, Sy, the resulting allocolloid being a sulphur 
sulphogel or a solid sulphosol. 


162 COLLOID CHEMISTRY 


egg albumen considerably less, and gum and starch 
but little action. The relative protective efficiency 
of these colloids parallels their protective action in the 
case of colloidal gold. 

An observation recently made by J. Alexander * is 
of interest here. Moissan (Comptes rend., 144, 598, 
J. 8S. C. I., 1907, 418) has noted that the addition of a 
little platinum to metallic mercury causes the latter to 
“emulsify” in water. Upon making up such an ‘‘emul- 
sion,’ Alexander noticed that the supernatant fluid 
remained turbid upon standing, and therefore examined 
the fluid in the ultramicroscope, which revealed the 
presence of colloidal metallic particles in active 
motion. 

Boiler Scale 

When water is purified before injection into the 
boiler, any precipitate is preferably made as coarse as 
possible so that it can be readily settled or filtered out. 
But where the precipitate is formed within the boiler, 
exactly the opposite result is sought. Most ‘‘boiler 
compounds” contain such soluble colloids as dextrin, 
tannin and bark extract, and some engineers put 
potatoes or starch in their boilers together with soda 
ash. 

Any precipitate formed in the presence of these 
colloids adsorbs them and tends to remain in a finely 
dispersed non-coherent condition, so that it is readily 
removed when the boiler is blown down. The forma- 
tion of hard, crystalline scale is thus prevented. 


Cement, Mortar, Plaster 
Freshly mixed cement and mortar contain colloidal 
sols and gels which not only tend to delay or prevent 
*J. S.C. 1. 1909, 28, 280. 


PRACTICAL APPLICATIONS 163 


the crystallization of dissolved substances, but also 
upon coagulation or setting bind the solid elements of 
the mixture into a coherent whole. 

According to recent investigations [see A. A. Klein 
and A. J. Phillips, U.S. Bureau of Standards, tech. 
paper 43 (1914); P. H. Bates and A. A. Klein, tech. 
paper 78 (1916)], Portland cement contains much 
tricalcium aluminate (3CaO.Al,.03), which is re- 
sponsible for its initial set, but which alone sets 
rapidly into a brittle crystalline mass. However, the 
cement contains dicalcium silicate (possibly an ad- 
sorption compound between lime and silicic acid), 
like that described by Le Chatelier [La Silice et les 
Silicates, 1914] as being formed by adding calcium 
hydroxide to dilute silicic acid sol,* which hydrates 
very slowly, forming an amorphous colloidal mass 
that evidently acts as a protector to the tricalcium 
aluminate, inhibiting its crystallization. Within 
about one day the tri-calcium aluminate begins 
its hydration and crystallization, which lasts about a 
week, and tends to make the cement weak. For the 
next three weeks, however, the main change is the 
progressive hydration of the lime-silica compound, 
with the development of a large amount of colloidal 
material which greatly increases the free surface and 
the strength. 

The addition of such salts as calcium sulphate and 
the like, which are largely used in cement as a retarder, 
is regarded by W. Michaelis as altering the solubility of 
the aluminates and silicates, and by Rohland as in- 
fluencing the coagulation rate of the colloids. Prob- 
ably both effects co-exist, and there exists in cement a 


*From the bulky gelatinous precipitate the lime may be entirely 
removed by washing. 


164 COLLOID CHEMISTRY 


condition of cumulative colloidal protection analogous 
to that found in glass, flour and metals. 

In the case of plaster of Paris the set may be greatly 
delayed by glue, gum and protective colloids, and re- 
tarders of this kind have been in use for many years. 
The following table shows the result of adding gelatin 
to plaster of Paris: 


One part 
water contain- Time to set Microscopic appearance of 
ing gelatin— in minutes. slide. 
Per cent. 
0 40 Characteristic crystals of calcium 
sulphate. 

1/100 50 No crystals, except in a few spots 
where some colloid-free solution 
had diffused out. 

1/10 260 No crystals.* 

1/4 510 No crystals.* 

1/2 960 No crystals.* 

1 Not set in 48 hr. No crystals.* 

2 Not set in 48 hr. No crystals.* 


* No regular, ordinary crystals, the mass consisting of aborted, modi- 
fied or sphero-crystals, or of colloidal crystals or other minute unoriented 
groups, or of a mixture of these. 


CHAPTER 14 


PRACTICAL APPLICATIONS (Continued) 
CHEMICAL ANALYSIS * 


In gravimetric methods the object of the analyst is 
to form a non-colloidal precipitate that can be sepa- 
rated on a filter, washed and weighed. Von Weimarn 
has shown that precipitates formed in very dilute or 
very concentrated solutions tend to be colloidal, 
and for this reason the analyst usually chooses solu- 
tions of medium concentration, together with an 
excess of the precipitating reagent as a coagulant, 
acid in some cases, alkali in others. Adsorption of 
soluble substances by precipitates is a prolific source 
of error, which the analyst minimizes by thorough 
washing and a choice of precipitants, concentrations, 
andsoon. The work of H. J. Weiser [J. Phys. Chem. 
1919, et seq.] indicates what large errors may be due 
to neglect of precautions against adsorption. 

Often when analytical methods speak of dissolving 
a precipitate, it is simply peptized or dispersed into a 
colloidal solution which passes through the filter paper. 
Thus the gel of aluminum hydroxide is readily pep- 
tized by alkali and that of iron hydroxide by hydro- 
chloric acid. 

Sometimes substances are added which inhibit 
precipitation by producing a colloidal non-filterable 
sol. Citrates and tartrates are especially apt to do 
this, and require special analytical treatment. The 
presence of such salts or of protective colloids (glue, 


* This topic has been treated at length by Prof. H. Bassett in 4th 
Report on Colloids ,etc., Brit. Assoc. Adv. Sci. 1922. 


165 


166 COLLOID CHEMISTRY 


dextrin, organic extractives) in technical products or 
specimens may lead to grave errors in analysis, so 
that the analyst should destroy them by oxidation or 
ignition, or else nullify their effects by a suitable excess 
of coagulant or precipitant. ‘‘Organic matter,” as itis 
vaguely termed, may act: (1) By totally or partially 
preventing the formation of precipitates; for ex- 
ample, tartrates prevent the precipitation of alumi- 
num, chromic and ferric hydroxides [Yoshimoto, J. 
Soc. Chem. Ind. 1908, 27, 952]; (2) by preventing the 
satisfactory filtration of a precipitate when formed 
[Mooers and Hampton, J. Am. Chem. Soc. 30, 805]; 
(8) by rendering precipitates difficult to wash and 
purify [Duclaux, J. Soc. Chem. Ind. 1906, 25, 866]. 

A few experiments will make this clear. Hydro- 
chloric acid gives with lead acetate solution a heavy 
coagulated precipitate, but with sodium chloride, a 
less highly ionized precipitant, only a colloidal pre- 
cipitate. If some glue solution be first added to the 
lead acetate, then sodium chloride produces no visible 
precipitate at all. In testing urine for sugar with 
Fehling’s or Benedict’s solution, as Prof. M. H. 
Fischer has pointed out, the simultaneous presence of 
albumin (or other protective colloid) is apt to bring 
down the copper oxide in a yellow colloidal state which, 
if some of the blue copper remains unreduced, ap- 
pears as a greenish turbidity. For this reason the 
urine is boiled and filtered before testing for sugar, to 
separate out any albumin. 

The Wassermann and precipitin tests are best under- 
stood on the basis of colloid chemistry—adsorption, 
peptization, protection or lack of protection, coagula- 
tion, and so on. The nomenclature of Ehrlich (com- 


PRACTICAL APPLICATIONS 167 


plement, middle-piece, end-piece, antigen, and so on), 
while appealing to the imagination, has no counterpart 
in the actual facts. 


Pharmacy and Therapeutics 


The pharmacist and his predecessor, the iatrochem- 
ist, have long utilized colloid chemical processes. 
Mercury is readily brought into the colloidal state by 
trituration with fats (blue ointment) and was largely 
used by Paracelsus (1493-1540 [?]), though known 
long before. Gold was also thought to have great 
healing virtues, and its colloidal solutions were known. 
Eau-de-vie de Danzig seems to be a relic of this 
practice. It contains gold leaf. Thus Chaucer, in 
making a sly hit at the physician, says: 

For gold in physic is a cordial; 
Therefore he loved gold in special. 

In making emulsions the pharmacist uses gum 
arabic, Irish moss, tragacanth, and the like. If ferric 
chloride be added to gum arabic emulsion of codliver 
oil, it coagulates the gum, and the oil, no longer pro- 
tected by the emulsostatic action of the gum, promptly 
separates out. Milk of magnesia may be kept in 
colloidal state by carbohydrate protectors. 

Colloidal silver (collargol, argyrol, and the like) 
is an excellent germicide in many cases and a pre- 
ventive of ophthalmia neonatorum. Colloidal sul- 
phur, artificially prepared and as ichthyol (a fossil 
sulphur-containing fish product), is valuable in skin 
troubles. Colloidal mercury (hygrol, blue ointment, 
and the like) is largely used. The introduction of 
gum arabic into the normal saline solution injected 
intravenously in cases of bleeding and surgical shock 
has saved many lives. It increases and maintains the 

12 


168 COLLOID CHEMISTRY 


blood pressure and viscosity. Ferric salts, especially 
the chloride which readily hydrolyzes into the hydrate, 
act as styptics by coagulating the blood colloids. 
Alum and antipyrine act similarly. 

On the other hand, citrates, oxalates, and hirudin 
(extracted from the head of the leech) tend to inhibit 
the coagulation of blood. 


Antiseptics and Bacteriology | 

Colloid chemical factors largely control the action 
of antiseptics.* To be effective a disinfectant must 
be adsorbed by the bacteria, and then it coagulates 
or peptizes their protoplasm, or else flocks them out. 
Natural antitoxins act similarly. Thus in the Widal 
test the agglutination of the typhoid germ is observed 
microscopically. Sodium chloride increases the effec- 
tiveness of phenol, an effect analogous to the driving- 
on action of salts in dyeing. Beyond a certain dose, 
mercury slays relentlessly. It is fixed by the kidneys 
and slowly causes an irreversible coagulation of the 
cell colloids. Lead is likewise irreversibly fixed in 
the body. 

Dilution may remove an antiseptic before its results 
are irreversible. 

Since bacteria are of the order of size of 1 uw, their 
motion is partially Brownian, though the unequal lib- 
eration both of gas and diffusing products is also a fac- 
tor. As Albert Mary has shown (Dictionnaire de Biol- 
ogie Physiciste, 1921) the tuberculosis bacterium 
(Koch’s bacillus) may become toxic by the selective 
adsorption of substances normally present in small 


*See H. Bechhold, “Colloids in Biology and Medicine,” trans. by 
J. G. M. Bullowa; E. K. Rideal, 5th Report on Colloids, etc., Brit. 
Assoc. Adv. Sci. 1923. I. 8S. Falk, Abstr. Bact., 1923. 


PRACTICAL APPLICATIONS 169 


amount in the blood (cholesterin). The extreme 
sensitiveness of bacteria to minute changes in acidity 
(py) shows how intimately their life is controlled by 
variations in the degree of swelling or dispersion or 
electrical charge of their constituent colloids. 

Rideal attributes the selectiveness of adsorption to 
the sub-microscopic inhomogeneity of the external 
bacterial surface, which has a checker-work of acid 
and basic areas. An extension of this idea justifies the 
now classic statement of van’t Hoff that enzyme and 
substrate are fitted like lock and key in cases where 
the enzyme acts (specific action). 


Biology and Medicine 


The changes which occur in most biological pro- 
cesses are remarkable, not only because of their pro- 
found nature, but also because they are produced 
rapidly, at comparatively low temperatures and in the 
presence of dilute reagents. With apparent ease the 
living organism disintegrates proteins, oxidizes carbo- 
hydrates and fats and synthesizes substances of great 
complexity. High temperatures and powerful re- 
agents which would be destructive of life are necessary 
to bring about changes of this character under ordinary 
laboratory conditions. 


Enzymes 


The high efficiency of the biological juices was 
once thought to be due to a mysterious ‘‘vital 
force,” but this idea was disproved by Buchner, 
who isolated from yeast triturated with sand, a lifeless 
zymogen or enzyme which had the power of converting 
sugar into carbon dioxide and alcohol. Enzymes 
(pepsin, diastase, pancreatin, papain, lipase and the 


170 COLLOID CHEMISTRY 


like) are colloidal catalyzers, whose existence seems 
to depend on the presence of adsorbed protectors; for 
the more they are purified the more unstable they be- 
come. They are coagulated and inactivated by 
shaking, heating, electrolytes and so on, all of which 
cause coagulation of colloidal solutions with a decrease 
in their free surface and in the activity of their par- 
ticles. The effective acidity (pq) of the solution in 
which an enzyme happens to be largely influences its 
action, probably by regulating its degree of dispersion 
and electric charge, which in turn affect free surface, 
adsorbability and kinetic activity. 

Enzymes appear to act by forming with the sub- 
strate a compound of indefinite and unstable character, 
probably, as Sir W. M. Bayliss suggests [‘‘ The Nature 
of Enzyme Action”’], a colloidal adsorption compound, 
which breaks down, liberating the enzyme again to 
continue the action. Croft Hill showed that the action 
of enzymes may be reversible, but in the organism 
reversion is prevented by the diffusion, dilution, or peri- 
staltic removal of the products formed. Where in- 
creases of concentration occur in cells, tissues or organs, 
synthetic processes may be induced by enzymes. 

Ultramicroscopically, enzymes show a multitude 
of colloidal particles in very rapid motion. Any sub- 
stance immersed in such a solution, providing its elec- 
tric charge or free fields of force permit the close ap- 
proach or concentration of the enzyme at its surface 
by adsorptive action, must undergo a terrific bom- 
bardment, and it seems likely that enzymes produce 
their effects by virtue of their specific surface actions 
(which is influenced by the hydrogen ion concentra- 
tions of their solutions) and by the motion of their 
particles. 


PRACTICAL APPLICATIONS 171 


To see if this idea could be experimentally verified, 
J. Alexander followed ultramicroscopically the action 
of diastase on starch grains and the action of pepsin 
on coagulated egg albumen. 

The diastase ultramicrons, maintaining their active 
motion, gradually accumulated about the starch gran- 
ules, which after a time showed a gnawed or ragged 
margin. The adsorption and motion of only the larger 
ultramicrons could be followed, but the bright appear- 
ance of the field indicated that numerous smaller par- 
ticles were present, and some of the intermediate size 
were seen. The albumen specimen was a dilute solu- 
tion of egg white, heated nearly to boiling. It was 
opalescent, and showed a field full of bright, rapidly 
moving ultramicrons which immediately clumped or 
coagulated into large, motionless masses upon the 
addition of a droplet of pepsin solution (Fairchild’s 
containing 15 per cent. of alcohol by weight). Upon 
‘‘activating”’ the pepsin with a droplet of decinormal 
hydrochloric acid, the large groups burst into small 
groups and isolated ultramicrons, which at once re- 
sumed their active ‘‘dance.’’ Soon, however, the 
albumen particles began to grow fainter and disappear, 
the field meanwhile becoming brighter and brighter, in- 
dicating that simultaneously smaller ultramicrons 
or amicrons were being formed. The additon of pep- 
sin to a test tube of the opalescent albumen solution 
caused it to clear gradually at room temperature. 


Cytology 
Physiological chemists have made illuminating re- 
searches into the chemistry of cell metabolism, and 
cytologists have, mainly by microscopical methods, 


172 COLLOID CHEMISTRY 


accumulated a wealth of interesting albeit confusing 
facts, the elucidation of which will lead us still nearer 
to an understanding of the mystery of life processes. 

When a germ cell undergoes mitosis (or karyokinesis 
as it is also known), some of the constituents of the 
nucleus arrange themselves into a long irregularly 
twisted thread or spireme (loose skein), which then 
becomes more open but shorter (open skein). The 
open skein splits into short segments, frequently 
U-shaped, and these divide equally on either side 
of an equatorial plane which separates the cell into 
two parts, each of which develops further into a 
separate cell. The formation of star-like polar centers 
(amphiasters) is one of the many other associated 
phenomena observed. 

Since the substances comprising the skein segment 
are readily stained by the microscopist’s dyes, they 
are called chromatin substances, and the structures 
are called chromosomes. Most heritable character- 
istics which have been studied are transmitted solely 
through the chromosomes, which vary both in number 
and character with different plants and animals. 

The problem of investigating the colloid-chemical 
changes underlying the microscopical picture and 
accompanying the chemical changes, is a most formi- 
dable one; but once it is closely envisaged, some 
vulnerable points will probably be found. A _ be- 
ginning has already been made by R. Chambers, 
A. Heilbronn, L. V. Heilbrunn, W. Seifriz and others. 
The micro-method of Freundlich and Seifriz (Zeit. 
phys. Chem. 104, 283-61) (which consists in pulling 
a tiny [18 » +] particle of nickel through a gel by 
an electro-magnet of known power) shows that the 


PRACTICAL APPLICATIONS LTS 


inner protoplasm of the unfertilized egg of an echino- 
derm (Echinarachnius parma) has about the viscidity 
of concentrated glycerin, while the peripheral layer 
has the consistency of a soft gelatin gel [Seifriz, 
Brit. J. Expt. Biol. 7, 431-42 (1924)]. 

At mid-mitosis the cytoplasm about each amphi- 
aster has the consistency of a still plastic gelatin 
jelly, while the peripheral protoplasm of the egg 
becomes as viscous as bread-dough. Following the 
completion of mitosis, the viscosity drops again, in 
preparation for another division. L. V. Heilbrunn 
investigated the viscosity of cellular constituents by 
centrifugal methods [J. Expt. Zool. 30, 211-87 (1920)]. 

If in the middle of mitosis (metaphase) the egg be 
subjected to slight pressure, the whole karyokinetic 
spindle-shaped structure collapses, resembling in this 
respect (as Seifriz points out) the gel of iron oxide 
described by Schalek and Szegvary [Kolloid-Z. (1923) 
82, 318-9] and that of metallic cadmium described 
by T. Svedberg (Rept. Farad. Soc. and Phys. Soc. 
Lond. on Phys. and Chem. of Colloids, 1921). 

Some of the colloid-chemical principles involved in 
intercellular changes are apparently the following: 

Gel formation (pectization)—when the gel forms in 
certain localities, banded orientation (Liesegang’s 
rings) and differential diffusion may occur in it. 

Re-solation (peptization)—this may be consequent 
upon the action of enzymes released or activated by 
changes in H-ion concentration, which may follow 
differential diffusion. ‘These changes in H-ion con- 
centration may also determine viscosity. 

Cumulative protection—alteration of one of a con- 
nected series of protectors may produce important 
changes. 


174 COLLOID CHEMISTRY 


Colloidal influence on crystallization tendencies— 
astral rays seem indicative of this. 


Growth 


The fact that structures simulating organic growths 
can be produced by allowing precipitates to form in 
colloidal gels has long been known, and many beauti- 
ful experiments may be made along these lines.* 
Differential diffusion through semi-permeable mem- 
branes, and the resulting osmotic pressure, are large 
factors in growth, to which must be added the forma- 
tion of insoluble chemical compounds and, especially 
in organisms, the manifold effects of colloidal pro- 
tection, coagulation, peptization, and adsorption. 
Thus Wislicenus showed that the cambial or running 
sap of trees in the spring is full of colloidal substances 
which coagulate irreversibly after their adsorption by 
the wood cellulose. 

The punctum vegetationis of plants is usually more 
acid than the balance of the plant (has a lower pq) 
and therefore tends to swell more and be more hydrous 
and tender—e.g. in the case of asparagus. 

The colloidal nature of the tissues and enzymes 
with which food comes in contact directs and regu- 
lates the whole process of development. From the 
same soil we may produce a lily and a beet, a turnip 
and arose. From tiny cells almost undistinguishable 
from each other there develop the most diverse ani- 
mals; yet compressed within that space are the dif- 
ferences in chemical constitution and colloidal struc- 


*See R. E. Liesegang, ‘‘Kolloid-chemische Theorie des Lebens”’; 
Stéphane Leduc, “Les Croissance Osmotique et l’Origine des Etres 
Vivantes’’; A. L. Herrera has worked over thirty years on Plasmogeny. 


PRACTICAL APPLICATIONS ilo 


ture that render possible and, in most cases, insure very 
tenaciously the close resemblance of offspring to parent. 

Constructive and destructive processes are going 
on side by side in the organism, and, as R. S. Lillie 
puts it [Scientific Monthly, Feb., 1922], visible growth 
simply represents the accumulated excess of the former 
over the latter. Broadly, the constructive processes 
involve the formation of species-specific sols and gels, 
followed or accompanied by the formation of pre- 
cipitates in their presence, and then usually by a slow 
syneresis which produces most of the hardenings of old 
age. Chemically, there is much similarity, if not 
sameness, about the structures formed. Thus even 
cartilage, the precursor of bone, is allied to chitin of 
insects and the mucus of snails, for all yield chon- 
droitin-sulphuric acid. But, physically, there are 
enormous differences in form, percentage of composi- 
tion and function. 

Colloidal sols and gels exert a powerful and specific 
influence on developing structures, as may be seen by 
allowing different salts and salt mixtures to crystallize 
in the presence of colloids such as gelatin or gum arabic, 
and also by allowing precipitates to form by diffusion 
in various jellies (gelatin, agar and silicic acid). 
Although the details are of unfathomed complexity, 
the specificity of organisms and of the fragments, 
buds, spores or seeds by which they reproduce, is due 
to specific colloids and to the substances the colloids 
adsorb or whose formation they direct.* 


Evolution 


It is the general rule in biology that descendants 
resemble parents, and that a parent organism cannot 


* See R. S. Lillie, ‘‘Protoplasmic Action and Nervous Action,” Univ. 
of Chicago Press, 1924. 


176 COLLOID CHEMISTRY — 


pass on to offspring a factor which the parent did not 
receive from the germ-plasm of its immediate pro- 
genitors. Many apparent exceptions to this general 
rule have been traced to the existence in the parent 
gametes of recessive factors, which, while suppressed 
in the parent, may be liberated again in the offspring. 
Whether we accept the view of Darwin that large 
differences can represent the summation of small 
differences, or the more probable view of Bateson and 
others, that mutation or variation is a definite physio- 
logical event, no satisfactory explanation has been 
given as to the origin or source of these exceptions 
to the general rule of resemblance, although they 
constitute the steps by which evolution haltingly 
proceeds. 

The crying need that we must find a chemical, 
physical or physico-chemical basis for mutation or 
variation has been voiced by many. Thus in his 
address before the British Association for the Ad- 
vancement of Science (Australia, 1914, reprinted in 
Smithsonian Report, 1915, pp. 359-394), Sir William 
Bateson says: ‘‘Every theory of evolution must be 
such as to accord with the facts of physics and chem- 
istry, a2 primary necessity to which our predecessors 
paid small heed. . . . Of the physics and chemistry 
of life we know next to nothing. Somehow the 
characters of living things are bound up in properties 
of colloids, and are largely determined by the chemical 
powers of enzymes, but the study of these classes of 
matter has only just begun. Living things are found 
by simple experiment to have powers undreamt of, 
and who knows what may be behind?” 

In nature, both animate and inanimate, the follow- 
ing basic factors tend to produce symmetrical orienta- 


PRACTICAL APPLICATIONS 177 


tion or aggregation: (1) Crystallization and orientation 
in surfaces; (2) Diffusion, as in the formation of 
Liesegang’s rings, agate, etc.; (8) Electric or magnetic 
fields of force; (4) Harmonious vibration as of air, 
water, etc. We here disregard mere chance and the 
conscious arrangement by man. 

The main factors modifying the crystallization of 
pure substances are: (1) Concentration; (2) Temper- 
ature; (3) Pressure; (4) Agitation; (5) The presence 
of other substances, especially of colloids, which may 
profoundly modify crystal forms by protective action; 
(6) Iso-colloidism. Some substances have the power 
of interfering with their own crystallization, because 
a portion, which first reaches the colloidal state, then 
protects the balance. 

Deviations from normal crystalline forms produced 
by the presence of colloids are usually symmetrical, 
but may not appear crystalline. Changes in the 
nature or degree of dispersion of the colloid, or in its 
percentage, mixtures of colloids, variations in salt or 
H-ion concentration* of the solution, must all have an 
effect on the resultant quasi-crystals. Enzymes may, 
of course, entirely change the nature of the colloid. 
The species-specificity of proteins seems to be main- 
tained by degenerating food protein to simpler forms 
(polypeptids and amino-acids) and then building up 
the specific proteins from these. 

Among the factors influencing diffusion, especially 
in gels, are the chemical nature and particle size of 
the gel, and the concentration and nature of the 
diffusing solution. In his chapter on ‘‘Growth, 


* NaCl crystallizes in cubes from acid solutions but in octahedra 
from alkaline solutions. 


178 COLLOID CHEMISTRY 


Metamorphoses and Development,’’ Bechhold (‘‘Col- 
loids in Biology and Medicine,” trans. by J. G. M. 
Bullowa, p. 252 et seq., D. Van Nostrand Co., 1920) 
refers to some of the remarkable diffusion figures and 
osmotic forms* produced by F. E. Runge and by 
Stéphane Leduc, some of which resemble alge, fungi, 
seaweed, etc., and even show a cellular microstructure. 
While pointing out the great differences between these 
formations and the organized structures they simulate, 
Bechhold says: ‘‘The physical forces which produced 
these inorganic formations are the same as those 
which produce the growth and configuration of organ- 
ized material membranes, osmotic pressure, diffusion.” 

Perhaps the most familiar instance of modified 
crystallization is to be found in the delicate frost 
tracery on window panes, the forms being probably 
influenced by the glass (itself a colloid) or by sub- 
stances adsorbed at its surface.j The writer has 
pointed out the powerful influence exerted by colloids 
such as gelatin, gum arabic and albumin on crystal- 
lization (Kolloid Zeit. 4, 86, 1909), and R. E. Liese- 
gang, looking at the question from the opposite stand- 
point, has described the power of crystalloids to give 
a form to colloidal jellies (Kolloid Zeit. 7, 96, 1910). 
It may be said that with different salts or combina- 
tions of salts, various colloids or combinations of 
colloids, and variations in concentrations, temper- 
ature and speed of evaporation, will produce charac- 
teristic and generally reproducible forms on a micro- 
scope slide. A characteristic form of sodium chloride 
is a four-pointed star with fern-like arms which cross 
at a slight angle. 


* A. L. Herrera has devoted over 35 years to these studies. 
+See, e.g., the figures given by Wilson J. Bentley in “Monthly 
Weather Rev.” 1907. 


PRACTICAL APPLICATIONS 179 


A few slides made with solutions of common salts 
such as NaCl, MgSO,, Na.SQu,, etc., containing from 
0.5 to 50 per cent. of gum arabic or gelatin (figured 
on the basis of the dry salt), will illustrate what is 
meant. When a drop of the mixed solution is allowed 
to dry on the slide without cover glass, changes of 
concentration and temperature occur, giving a field 
that changes progressively from rim to center of the 
drop. A solution of one part sodium chloride, one 
part sodium carbonate (dry) and one tenth part gum 
arabic or gelatin in ten parts of water, when dried, 
shows in some part of the field a ‘‘flowering plant,”’ 
with graceful stems and characteristic four-petaled 
flowers. 

To see that a marked change may be produced by 
modifying the colloidal state of the protective sub- 
stance, a slide was made with a solution containing 
egg albumen as the colloid. The solution was then 
heated until the albumen began to show a milkiness, 
another slide was made, and after drying was com- 
pared with the first unheated specimen. The differ- 
ence in crystallization was considerable. 

Unusual crystalline forms, such as sphero-crystals 
and sheaf-like groups which are so often seen in the 
crystals of substances derived from organisms, are 
very often consequent upon the protective action of 
- some colloid from which they are not entirely purified. 
Another curious occurrence must be mentioned here, 
which may be termed auto-protection because it is due 
toiso-colloidism. Before reaching the ordinary visibly 
crystalline state, particles of every substance must 
pass through the colloidal zone, and the particles 
first reaching that state may interfere with the normal 


180 COLLOID CHEMISTRY 


crystallization of the rest. Thus ammonium salts, 
even without the addition of protective colloids, are 
prone to assume feathery or fern-like forms. The 
phenomenon is marked in the oleates and is probably 
the underlying cause of the formation of myelins, 
although their formation is fostered by such lipoid 
protectors as cholesterin. According to J. G. Adami 
(Harvey Society Lecture, 1906), if certain simple soaps 
be dissolved by warming on a slide with water and 
then allowed to cool, they may show upon examina- 
tion in the polarizing microscope a perfect rain of 
doubly refracting spherules, which, depending on the 
nature of the soap, may last for hours or days or else 
immediately give place to a brilliant white layer of 
formed crystalline plates. The fluid crystals of O. 
Lehmann are probably examples of auto-protection, 
and W. B. Hardy, E. Hatschek and others have 
described substances which form unstable gels that 
soon become crystalline (see p. 45). 

The bio-colloids are so readily affected by salts, 
H-ion concentration (effective reaction), temperature, 
actinic (sun’s rays) and traumatic (shaking, me- 
chanical injury) effects, that it is more surprising that 
plants and animals should breed true than that they ~ 
should show variations. Therefore, although indi- 
viduals may be much affected by such changes during 
their lives, it is evidently a rare occurrence that these 
changes are registered in the germ plasm by which 
alone they may be transmitted to offspring. The 
specificity of the germ plasm is evidently guarded by 
many factors, among which seem to be selective ad- 
sorption and differential diffusion of dissolved sub- 
stances through its protecting walls or membranes. 


PRACTICAL APPLICATIONS 181 


Nevertheless unusual influences must occasionally 
change it materially without destroying it, and along 
this line experiment may be directed. It may be 
that the germ plasm can be affected through ‘the 
somatoplasm, as well as by direct means. And of all 
the variations, in nature only the beneficial changes 
survive. 
Physiology and Pathology 


The bio-colloids consist of carbohydrates (starch, 
cellulose, glycogen), proteins (albumins, globulins, 
ossein) and lipoids (lecithin, cholesterin, fats and oils). 
Each tissue has a normal state of swelling or turgor, 
which is greatly influenced by the effective reaction 
(acid if py < 7, alkaline if p, > 7) and by salts. 
The swelling is least at or near the isoelectric point; 
its Increase by both acids and alkalis soon reaches a 
maximum, after which it falls again. Neutral salts 
oppose the action of acids and alkalis, apparently by 
diiving back their ionization, and thus bringing the 
effective reaction (H-ion concentration, p, value) 
nearer to the isoelectric point. So sensitive are col- 
loids to minute changes in reaction detectable only 
potentiometrically or by the use of very delicate indi- 
cators, that fibrin will easily distinguish by a big dif- 
ference in swelling, between conductivity water and 
ordinary distilled water, which contains CO, and has 
& Py Of about 5.5. | 

If the oxidation processes of the body are normal, 
the hydrogen in foods is oxidized mainly to water, 
and the carbon mainly to carbonic acid—a gaseous 
acid which is exhaled without demanding protein or 
fixed alkali of the organism for its elimination. Nearly 
two pounds of caustic soda would be required to: 


182 COLLOID CHEMISTRY 


neutralize the acid produced daily by the average 
adult. In cases of pathological oxidation, as in 
diabetes, dextrose (glucose) accumulates in the blood 
beyond the normal average of about 1 per cent. 
(hyperglycemia), exerting a dehydrating or shrinking 
action on the tissues, while fats are oxidized only 
partially to the poisonous acetone, diacetic acid, and 
beta-oxybutyric acid, which may produce “acidosis,” 
really a diminished alkalinity recognizable by the 
fact that an abnormally large quantity of sodium 
bicarbonate is needed to render the urine alkaline. 
As the body colloids are thus forced nearer to their 
isoelectric point, their water-holding capacity dimin- 
ishes, ‘‘free’? water appears in the blood, and the 
patient shrinks. In fact, throughout life there is a 
continual syneresis of the body colloids: compare 
the chubby hand of an infant with the shrunken hand 
of an old man. 

Local accumulation of acid may cause swelling 
(edema); for example, insect stings which may be 
simulated by ‘‘stinging”’ gelatin with a needle dipped 
in acid. If acid accumulates in an organ, having a 
rigid capsule (eye, kidney), the swelling tends to 
establish a vicious circle (glaucoma, nephritis) by 
compressing the blood vessels and cutting down the 
alkaline blood stream, which is unable to absorb and 
wash out the acids (mainly CO,) formed by living pro- 
toplasm. The degeneration of the kidney in these 
circumstances is so typically one of the colloidal 
swelling and coagulation that the physician judges the 
condition by the microscopic appearance of the casts 
(hyaline or granular) sloughed off by the tubules. 
Certain infections (scarlet fever, for example) produce 


PRACTICAL APPLICATIONS 183 


substances likewise harmful to the kidney tissue. 
[See ‘‘Edema and Nephritis,” by M. H. Fischer.] 

The relation between the clinical, pathological, and 
colloid chemical pictures or complexes in inflammation 
has been diagrammatically illustrated by H. Schade 
(Miinchener med. Wochenschr. 1924, No. 1, p. 1-4) 
as follows: 














HyYPER- 
[Inflammation| Se ee eee 
| PLETHORIA | 
2 —_—--_— pdt i ah dette ok 
Ageing aE Hyperthermia Calor 
Processes Dol 
f i olor 
Exudative Hypertonia 
Processes SEE —| Tumor 
Proliferating H-Hyperionia = Rubor 
Processes ? Functio laesa 
CELLULAR MOLECULAR CLINICAL 
PATHOLOGICAL PATHOLOGICAL PICTURE IN 
PICTURE PICTURE SICKNESS 





Owing to increase in quantity of dissolved sub- 
stances, the osmotic hypertonia may reach as high as 
11 atmospheres, while H-hyperionia may result in a 
hydrogen ion concentration 50 times normal.* 


Capillary Circulation 


The remarkable work of A. Krogh and his collabo- 
rators}| shows that for the most part the capillaries 
intermittently open and close. They thus act practi- 
cally as tiny railroad sidings, along which the red 
blood corpuscles are shoved in single file like tiny 
trains of freight cars. When the motion stops, oxygen 
diffuses slowly into the surrounding tissue served by 


* Note.—This means a pg of between 5 and 6, which is about the 
acidity of ordinary distilled water not protected from CO: absorption. 
7 See “‘The Anatomy and Physiology of Capillaries.” 
13 


184 COLLOID CHEMISTRY 


the capillary, while carbon dioxide diffuses about 30 
times more rapidly to the red blood corpuscles. When 
the exchange reaches the proper balance, the capillary 
opens, and the arterial pressure forces the little freight 
cars, now loaded with carbonic acid, into the venules, 
their places being taken by new oxygen-containing 
corpuscles. 

Underlying these apparently simple microscopically 
visible changes are many factors, chief of which seem 
to be the following: 

(1) Hormonal or Endocrine Control. The posterior 
lobe of the pituitary yields to the blood a very potent 
hormone which exercises capillary control. Krogh 
thinks it regulates the tone of the tiny Rouget cells 
which embrace most capillaries; for in the frog, ex- 
tirpation of this lobe causes an edema which he says is 
due to the relaxation of the Rouget cells. Krogh 
estimates that this hormone is effective in less than 
one part in one hundred million, while Abel has 
isolated a purified hormone said to be effective one 
part in 18,750,000,000. 

(2) Nervous Control. ‘There exists both a psychic 
nervous control (‘‘blushing,” turning pale), and a 
vasomotor (sub-conscious) control. Light exercises a 
lasting effect on the reactivity of capillaries to stimuli 
(work of Finsen). 

(3) Turgidity Control. A variation in the degree 
of swelling or turgidity of the tissue colloids com- 
prising the capillary wall or surrounding the capillary, 
would obviously result in the opening or closing of 
_ the capillary and it would seem easy to explain the 
process by assuming that when the tissues accumulate 
carbonic acid, they swell and close down on the 


PRACTICAL APPLICATIONS 185 


capillaries, relaxing again as oxygen comes in and 
carbonic acid diffuses out to the red blood cells. The 
tissues would thus automatically regulate their supply 
of blood in accordance with their demands; and in 
fact Krogh found that in exercising muscle many 
more and larger capillaries are open than. in quiescent 
muscle. If anything prevents the capillary from 
opening again at the proper time, we have a vicious 
circle—an ‘‘acid’’ capillary surrounded by ‘“‘acid”’ 
tissue—leading to congestion or inflammation. 

Krogh states (loc. cit., pp. 131-133) that change in p, 
(H-ion concentration) does not seem to be the domi- 
nant factor in capillary control but that this control is 
associated in some way with a lack of oxygen. Krogh 
however estimates the p, from the CO, tension, a 
method open to objections because of the many other 
factors which may influence the H-ion concentration 
of tissue, e.g. the presence of buffer salts and proteins, 
and selective diffusion between the plasma and cor- 
puscles. Eliot R. Clark and Eleanor L. Clark 
[Anatomical Record, Vol. 27, p. 200 (1924)] found 
capillaries in a tadpole’s tail which probably did not 
lack oxygen because of the thinness of the tissue, but 
which nevertheless automatically opened and closed, 
as did also segments of capillaries between Rouget 
cells. 

It seems difficult to determine these microscopic and 
minute fluctuations in p, but a technique may yet be 
developed. As Krogh says (p. 134), it is ‘‘a case for 
renewed investigation”; but it is hard to forego the 
belief that variation in the swelling of tissue, associated 
with fluctuation in H-ion concentration and the 
amount of products of metabolism, is a big factor in 
the control of capillary circulation. 


186 COLLOID CHEMISTRY 


Psychiatry 

The proper functioning of the brain and the nerves 
depends upon the maintenance of their constituent 
colloids within certain limits of swelling and disper- 
sion. As is usual, the word certain is used to cloak 
our ignorance of the exact conditions, for the question 
is of great and as yet unfathomed complexity. 

Broadly speaking, in most mental disorders there is 
a preliminary reversible stage, which, if not taken in 
time, may lead to later stages which are partially or 
totally irreversible. As the physico-chemical changes 
of many of the bio-colloids and proteins (e.g., gelatin 
and albumin when heated) show first reversible 
changes which gradually become irreversible as the 
coagulative or alterative influences persist or increase, 
it igs natural to inquire whether incurable mental 
disorders may not be consequent upon irreversible 
changes in some of the brain colloids. When the 
psychiatrist says that a neurosis has become a psy- 
chosis, he is only recognizing a clinical fact without 
even hinting at the underlying causes. 

Since the blood supply throughout the body is sub- 
ject to nervous and endocrine or hormonal control 
(see paragraph on Capillary Circulation), and since 
the endocrine glands (suprarenal, pituitary, thyroid, 
etc.) may, by nervous stimulation and variation in 
their blood supply, pour their powerfully acting 
secretions into the blood stream, it is evident that 
nervous excitement or shock may affect the whole 
body most powerfully. Thus Crile bound a rabbit 
with tape so that it could not move, and allowed a 
tethered dog to bark at it. The rabbit died of 
‘fright,’ and its brain showed extreme congestion, 


PRACTICAL APPLICATIONS 187 


probably due to the excessive blood pressure con- 
sequent upon activated secretion of adrenalin, etc., 
without the possibility of any corresponding motor 
activity to act as a natural relief valve. 

Dr. A. J. Justschenko (‘‘The Nature of Mental 
Diseases,’’ Dresden, 1914) states that the viscosity of 
the blood is usually increased in dementia precox, 
senility, and maniac-depressive insanities, especially 
during the periods of excitement. Auguste Lumiére 
found that the blood of an epileptic, taken before a 
seizure and injected into a rabbit, produced epilepsy 
in the animal. The active part of the serum settled 
on standing and could also be centrifuged out, indi- 
cating that it is a colloidal flocculate of some sort. 


Anaphylaxis and Immunity 


A. Lumiére has also shown that, whereas extremely 
fine precipitates of inert substances (e.g., barium 
sulphate) are harmless when injected intravenously 
when freshly made, if the precipitate is aged it pro- 
duces serious disturbances whose gravity depends upon 
the size of the flocculates. The older and more 
aggregated precipitates cause violent shock and death. 
Although the organism tends to protect itself against 
the formation of coarse coagula by the principle of 
plural colloidal protection (see p. 43), if the protective 
colloidal chain is attacked and broken, the formation 
of flocculates of injurious sizes may occur, and as 
these are selectively adsorbed and block the capillaries 
in various places, they may cause a whole train of 
ills, depending on the locus and the degree of action.* 

We may understand immunity as due to an antigen 


* It is believed that some bacteria (anthrax) and spirille (spirochete 
pallida) act largely by mechanically obstructing the circulation. 


188 COLLOID CHEMISTRY 


which has gradually, by selective adsorption, sur- 
rounded itself with a protective colloidal layer of 
sufficient quantity and quality that it may take up 
and render harmless a succeeding quantity of antigen, 
or else insure its agglomeration as a colloidal sol. 
Thus, the ultramicroscope shows that tetanus toxin 
and tetanus antitoxin mutually precipitate each 
other, as do diphtheria toxin and antitoxin. Where 
the absorbed protective layer is deficient in quantity 
or quality, we may have a hypersensitization to pre- 
cipitation, so that a small succeeding dose of antigen 
produces a coagulation in masses sufficiently large to 
cause capillary embolism. In this manner we can 
understand the condition known as anaphylaxis and 
its occasional consequence, anaphylactic shock. 


Healing of Wounds 


In general this process depends on the formation 
of a fluid exudate or transudate (sol) which later sets 
to a gel, cementing together the severed surfaces and, 
on desiccation and shrinking, forming scar tissue. 


Digestion 

Crystalloids, such as sugars and salts, are diffusible 
through the semipermeable walls of the digestive 
tube, but fats and most colloidal foods must first 
be dispersed into a sufficiently fine state. The dis- 
integration of proteins (meat, egg white) and of 
carbohydrates (starch) is effected through enzymes 
(pepsin, ptyalin, trypsin); while fats (either directly 
or after separation of their glycerin) are emulsified 
by the alkaline pancreatic juice. The presence of 
protective colloids such as mucin, cholesterin (in bile), 
and the like may account for some of the divergences 


PRACTICAL APPLICATIONS 189 


between the behavior of natural and artificial di- 
gestive juices. Thus, membranes moistened with 
cholesterol permit the passage of particles to which 
they would otherwise be impervious. Indeed since 
all body fluids contain protective colloids, no chemical 
action can go on in the organism without being in- 
fluenced by their presence. 


Absorption, Secretion, Excretion 


These are largely the results of selective adsorption 
by, and differential diffusion through, the colloidal 
tissues and fluids, accompanied by swelling and 
shrinking. Thus the blood is a circulating colloidal 
fluid which holds more water when in the ‘‘acid”’ or 
venous condition than it does when in the more alkaline 
or arterial condition. Tissues and organs well supplied 
with venous blood tend to absorb water (intestine) ; 
whereas those well supplied with arterial blood tend 
to give up (secrete, excrete) water (kidney). Since 
the blood is passing in a continuous stream, the 
process continues as long as the supply of water 
permits, or until the blood is in equilibrium with the 
tissues. 

Conditions which decrease the capacity of the blood 
and tissues to hold water (diuretics, hyperglycemia 
and acidosis in diabetes) naturally result in the elimi- 
nation of the excess or ‘‘free’’ water (polyuria, diar- 
rhea). 

Minute quantities of acid increase the swelling 
capacity of colloids, which quickly reaches a maximum; 
after which increasing acidity causes shrinking. Neutral 
salts oppose the action of acids apparently by driving 
back the ionization of the acid and thereby reducing 
the H-ion concentration which is a controlling factor. 


190 COLLOID CHEMISTRY 


The functioning of organs is largely controlled by 
nervous influences. Thus, a sudden nervous shock 
by causing vaso-dilation may send an excessive supply 
of arterial blood through the mesenteric arteries (in 
effect an ‘‘internal blush’’), and cause secretion into 
the intestine (nervous diarrhea). 

Hydrolyzable salts are readily split up upon diffu- 
sion through a colloidal gel. Thus, ferric chloride 
upon diffusion through agar or gelatin gel yields ferric 
hydroxide which is selectively held back and hydro- 
chloric acid which differentially diffuses ahead. If 
a little potassium ferrocyanide and alkaline phenol- 
phthalein be added to the colloid gel, the latter will 
soon be banded in patriotic red, white, and blue (see p. 
49). Depending upon the structure of an organ, the 
location of its cavity, its arteries and veins, and its 
outlet, it may yield an acid secretion (gastric juice) or 
or an alkaline secretion (pancreatic juice). Individual 
substances or mixtures in the blood stream or other 
body fluids may be selectively separated or con- 
centrated. 

An insight into the complexity of these processes 
may be had by considering the functioning of the 
kidney. Its Malpighian tufts are plentifully sup- 
plied with arterial blood containing ‘‘free’”’ water; 
but since the interior diameter (lumen) of the afferent 
vessels is less than that of the efferent vessels, a “‘ back 
pressure” is produced which the heart-throb makes a 
pulsating pressure, causing the tufts to act as an 
ultra-filter and pour a copious but very dilute ultra- 
filtrate of the blood into the convoluted tubules. The 
tubules, however, have a plentiful supply of venous 
blood, which is unsaturated with water and reabsorbs 
most of it together with some of the substances con- 


PRACTICAL APPLICATIONS 191 


tained in the preliminary secretion; so that the con- 
centrated excretion (urine) that finally drips into the 
pelvis of the kidney has in solution many substances 


Convoluted 
Tubule , 


NAS ED 2y (LE: 
ate) MY 
We CNS as 6 S- 


edehtae, J nes ee ei. 
LONI AG Si. has ie 


Vascular 


exus 
(Glomerulus} 






i<--- Bowman’s 
Capsule 


PY 


Afferens NN 
[N 


We--Twig of Renal Artery 


Fig. 6. Glomerular Structure. 


found in the blood, but in totally different concentra- 
tions. Bechhold estimates that the average of two 
liters of urine voided daily by a man represents a 
preliminary secretion of fifty liters, forty-eight being 
reabsorbed within the kidney itself. 


Diagnosis 

Among diagnostic methods involving colloid chemi- 
cal principles may be mentioned the Wassermann, 
colloidal gold, colloidal mastic,* and formol tests for 
syphilis, and the Widal test for typhoid. The differ- 
entiation between casts and cylindroids, usually 
difficult with the microscope, becomes relatively easy 
in the ultramicroscope or dark field. (See Alexander 
and Connelly, Science, July 27, 1923.) 

* See e.g. Adler and Sinek, Klin. Wochschr. 2, 2071 (1923). 


192 COLLOID CHEMISTRY 


Chemo-Therapy and Colloid-Therapy 


We will never understand just how many remedies 
work until we trace their effects on the body colloids— 
a most formidable task. Many remedies operative 
in vivo do not work in witro. 

All diseases are caused by, or involve, changes in 
the body colloids which, though delayed by the 
presence of protective substances, are sometimes irre- 
versible. To cure the disease, we must remove the 
cause and aid the tissues and body fluids or ‘‘ humors,” 
as they were called, to return to their normal state of 
dispersion and swelling. Bacteria and other invading 
organisms, as well as the body tissues, have highly 
specific adsorptive powers, and the ‘‘shot-gun’’ method 
of try, miss, and try again is our main reliance in 
finding something that will kill the germ or disease 
and spare the patient. Salvarsan was originally 
known as ‘‘606”’ because that was its number in 
the long series of compounds Ehrlich was testing 
against certain resistant organisms. Quinine, long 
ago known and used by South American savages, 
is still our specific against malaria.* ‘‘ Bayer 205,” 
non-operative ‘‘in vitro,” is said to kill the trypano- 
somes that cause sleeping sickness. : 

Many old remedies, relegated to the scrap-heap, 
had virtues we are beginning to understand, and they 
are coming into use again. Thus kaolin (clay) is 
largely being used as a cataplasm and internally as an 
adsorbent. Some varieties of clay adsorb bacteria 
(cholera). We laugh at Chinese or Egyptian recipes 
involving caterpillars, toads and the like, and even 
at the toad-skin ointment used by early American 


* Speke and Burton (about 1850) reported that the Congo natives 
attributed malaria to the mosquito and sleeping sickness. The Zulus 
called malaria ‘‘ mosquito sickness.” 


PRACTICAL APPLICATIONS 193 


settlers, forgetting that toad-skin contains adrenalin 
(or an equivalent) which we accept today as one of 
the endocrines. 

Charcoal is another colloidal adsorbent in common 
use; it is sometimes impregnated with drugs which it 
yields gradually, and it may serve as an antidote by 
adsorbing strychnine, for example. Freshly precipi- 
tated (colloidal) iron hydroxide was introduced by 
Bunsen as an antidote for arsenic; milk, egg white, and 
other proteins are used for corrosive sublimate poison- 
ing; gruels or other mucilaginous drinks against aloes, 
cantharides, and the like. Dusting powders cool by 
increasing the free evaporative surface of the skin. 


Serum-Therapy 

Foreign proteins (antigens) when introduced into 
the blood tend to surround themselves by adsorption 
with a protective film which, after a certain period, 
acquires in most cases the ability to adsorb, and hence 
nullify the effects of a subsequent dose of poison, thus 
conferring active immunity. Or the immune serum 
may be formed in a horse or other animal and injected 
(passive immunity). With some individuals the first 
doses of antigen accumulate so small a protective layer 
that the resulting product is more readily coagulated. 
Sensitizations of this kind may cause anaphylazis, 
asthma, and so on (see p. 187). 


Plants 
Plant tissues are likewise colloidal gels and sols, 
and as the sap circulates through them each tissue 
selectively adsorbs and elaborates certain constituents. 
Thus, starch-forming substances are fixed by the roots 
of the potato and tapioca, by the stem pith of the 


194 COLLOID CHEMISTRY 


sago palm and by the seeds of cereals. The stems 
take up the substances required for upward growth, 
the roots those for downward growth. 


Bio-Electric Currents 


Most tissues have a potential against other tissues, 
the one having the higher solution pressure forming 
the anode.* That solution pressure is partially de- 
pendent on degree of subdivision was shown by Hulett, 
and is also evident from the fact that strained and 
unstrained metals have a potential against each other, 
as do injured and uninjured tissues. 

The conduction of nervous impulses (pain, sight, 
hearing) involves chemical changes resulting in electric 
currents, perhaps due to the liberation of electrons. 
But though we may trace the chemical or electrical 
effect on the brain, how this becomes thought or 
consciousness is still an impenetrable mystery ex- 
pressed in the old philosophical circle—‘ What 1s 
mind?” ‘“Nomatter.” ‘‘Whatis matter?” ‘Never 
mind.” 

When we consider the great variety of bio-colloids 
and their susceptibility to change under physical and 
chemical stimuli with resulting changes in their be- 
havior as adsorbents and diffusion media, we can 
understand why an almost infinite number of reac- 
tions may go on within their recesses, as they swing 
the balance of the law of mass action over particles 
reduced to a reactive degree of dispersion. 

* See work of R. S. Lillie. 


BIBLIOGRAPHY 


The following are some of the more important standards of reference: 


ENGLISH 

H. BrecuHo.p, ‘Colloids in Biology and Medicine” (trans. by Dr. 
J. G. M. Bullowa). 

E. F. Burton, “The Physical Properties of Colloidal Solutions.” 

M. H. Fiscumr, ‘‘Cidema and Nephritis.” “Fats and Fatty Degenera- 
tion.” 

H. N. Hoitmss, Laboratory Manual of Colloid Chemistry. 

Wo. OstwaLp, “Theoretical and Applied Colloid Chemistry”’ (trans. 
by Dr. M. H. Fischer). 

Wo. Ostwatp, “An Introduction to Theoretical and Applied Chem- 
istry’’ (trans. by Dr. H. M. Fischer). 

T. SvepBerG, ‘‘ Colloid Chemistry.” 

WILLOWs AND HatscHeEk, “Surface Tension.” 

Zstamonpvy, “Colloids and the Ultramicroscope” (trans. by J. Alex- 
ander). 

ZsiamMonpy, ‘‘Chemistry of Colloids” (trans. by E. Spear). 


FRENCH 


Cotton ET Mouton, “Les Ultramicroscopes et les objets Ultramicro- 
scopiques.”” 

Paut Gastou, “L’Ultramicroscope dans le Diagnostic Clinique et les 
Recherches de Laboratoire.”’ 

J. Perrin, ‘‘ L’Atome.”’ 

A. Lumiere, “L’Anaphylaxie,” “Theorie colloidale de la Biologie et 
de la Pathologie,”’ ‘‘Réle des Colloides chez les Etres vivantes.”’ 


GERMAN 


ArtTHuR M@t.uer, “Allgemeine Chemie der Kolloide.”’ 
Wo. Ostwa.p, “Grundriss der Kolloidchemie.”’ 

THE SvepsBeERG, “ Herstellung Kolloider Losungen.”’ 

H. Frevunv.icu, “ Kapillarchemie.”’ 

J. VAN BEMMELEN, ‘Die Absorption.” 

R. Zstemonpy, German originals of above translations. 


The ‘Zeitschrift fiir Chemie und Industrie der Kolloide (Kolloid- 
Zeitschrift)”? and “ Kolloidchemische Beihefte,’”’? published by Wo. 
Ostwald, are mines of information, containing both original articles 
and references, 

195 


196 COLLOID CHEMISTRY 


Abstracts of, or references to, practically all current articles and books 
on Colloid Chemistry are to be found under the division ‘Physical 
Chemistry”? of ‘‘Chemical Abstracts,’ published by the American 
Chemical Society. Furthermore, in the books above referred to, 
are to be found numerous valuable references. 

The National Research Council has published a large Bibliography 
of Colloid Chemistry prepared by H. N. Holmes, and the British Asso- 
ciation have already published five Reports on Colloids and Their 
Applications. 


GLOSSARY 


For the convenience of the reader, most of the less common terms 
employed by colloid chemists are here collected and briefly defined. 
absorption—passage of a liquid into a solid or a gel by the action of 

capillarity; e.g., water by a sponge, ink by blotting paper. 
adsorption—binding of a finely divided substance (a gas, liquid or col- 
loidal dispersion) to an interface. 

allocolloid—one allotrope colloidally dispersed in another, e.g., as with 
sulphur. 

amicron—a particle invisible in the ultramicroscope, usually less than 
about 5 wy in diameter. 

colloid—any substance when dispersed into particles whose sizes may 
vary between the approximate limits 5 wu and 100 wu. The 
individual particles may be tiny crystals, or haphazard, random 
or amorphous groups; or mixtures of the two may coexist. 
Fineness of dispersion is the critical factor, but the effects of 
dispersion extend beyond the limits stated. 

deflocculation—dispersion of aggregates or flocculates. 
emulsion—a fine dispersion of one liquid in another. The dispersed 
particles may or may not be of colloidal size; if not they usually 
form a cream layer on standing long enough. 
emulsoid—a, colloid in which both phases are a liquid. 
gel—an aggregation of colloidal particles into practically non-motile 
larger groups. Internal readjustment may cause syneresis (q.v.). 
hydrophile—iiterally “‘water-loving’’; applied to colloids which tend to 
remain dispersed, hold water or remain in solution (see lyophile). 
hydrophobe—literally ‘“‘water-hating’’; applied to colloids which tend 

to aggregate and shed water, or fall out of solution (see lyophobe). 
hysterests—time-lag in the aggregation of colloidal particles. 
interface—the surface between two phases. 

irreversible—a, colloidal dispersion that will not redisperse after desicca- 
tion at ordinary temperatures. 

isocolloid—a substance colloidally dispersed in itself, e.g., as colloidal 
ice in water. 

Liesegang’s rings—bands or rings usually regularly or rhythmically 
placed, which form when precipitates are produced by diffusion 
in jellies or similar structures. 

lyophile—literally ‘‘solution-loving”’; same as hydrophile, but also 
applicable to dispersion media other than water. 

197 


198 GLOSSARY 


lyophobe—literally “solution-hating’’; same as hydrophobe, but also 
applicable to dispersion media other than water. 
; 1 
micron (»)— 1,000 mm. 
weer ls 1 
milli-micron (up)— 1,000 = 1,000,000 mm. = 10 A. 


orientation—disposition in space or on a surface, relative to certain 
points, lines or surfaces. Opposed to molecular chaos. 
pectization—gelatinization; an aggregation of colloidal particles into 
masses of sufficient size to be substantially devoid of Brownian 
motion, the resulting mass forming a flocculent precipitate; 
gel, or jelly, according to conditions. 
peptization—the reversal of pectization; the dispersion of a solid colloid 
by an enzyme (e.g., pepsin, whence the term) or by simple action 
of a dispersing agent (e.g., of FeCl; gel by traces of HCl, SiOe 
gel by traces of NaOH), or by heat (gelatin). 
protection—inhibition of aggregation. 
protector—a substance, usually a reversible colloid, which prevents or 
tends to prevent the aggregation of colloidal particles. 
reversible—a reversible colloid is one that redissolves after desiccation 
at ordinary temperatures. 
sol—a colloidal dispersion whose particles have sufficient Brownian 
motion to keep them in solution. The dispersing medium is 
often employed as a prefix: e.g., 
hydrosol = in water 
sulphosol = ‘* H2SO, 
aleosol = ‘‘ C,H;OH 
aerosol = “ air 
pyrosol = “ a melt (glass, metal, etc.). 
specific surface—external, free, or rind surface per unit wt. 
submicron—ultramicron. 
suspensoid—a colloid in which the dispersed phase is a solid. 
syneresis—aggregation of particles in a gel, with incidental liberation 
of liquid. 
ultrafiltration—filtration (usually under pressure) through very close 
pored gels or jelly septa. 
ultramicroscope—a compound microscope having a dark field and means 
of intense illumination of the object to be examined by a beam 
of light preferably at right angles to the optical axis of the mi- 
croscope. Particles are thus seen by the light they diffract. 
ultramicron—a sub-microscopic particle visible in an ultramicroscope. 


AUTHOR INDEX 


AcHESON, E. G., 94. 
Apamt, J. G., 180. 
ADLER, 191. 


ALEXANDER, J., 18, 46, 64, 78, 81, 
88, 90, 101, 114, 119, 131, 135, 
139, 148, 160, 162, 171, 191, 195. 


ARCHER, 144 
ARRHENIUS, S., 60. 
ASTON, 8. 

ATTERBERG, 70. 
AUERBACH, R., 18, 87. 
BaAapDBE, 60 

BAuNTJE, 161. 
Bancroft, W. D., 64, 66. 
Barus, Cart, 65, 68, 69. 
BASKERVILLE, CHARLES, 107. 
Bassett, H., 165. 

Batss, P. H., 163. 
BaTESON, Sir Wo., 176. 
Bayuiss, Sir W. M., 170. 


BrecuHotp, H., 40, 41, 47, 48, 


77, 168, 177, 178, 191, 195. 
BEHREB, 31. 
Brtuamy, H. F., 140. 
BEneEpDIcKS, 146. 
BENTLEY, W. J., 178. 
BERZELIUS, 38. 
Bitz, J., 41. 
BiscuHorr, 127. 
Bock, 25. 
Bovyoucos, 76. 
BrepiaG, G., 26. 
BRIDGMAN, ’P. W., 14. 
BRIGGS AND SHANTZ, 77. 
Brooks, B. T., 108. 
Brown, Rosert, 100. 
BicHner, E., 169. 
Buiitowa, J. G. M., 41, 131, 178. 
BUNSEN, 29, 
BUREAU OF STANDARDS, 149. 
Burton, E. F., 52, 195. 
Buxton, 41. 
Catiow, 85. 
CAMERON, 74. 
CAMPBELL, THOS., 
CARPENTER, H. Cc. Tf, 153, 157. 


14 


CuHaucer, G., 167. 

CHINA, 25. 

CHRISTMAN, 94. 

Cuark, ELeaAnor L., 185. 
Cuark, Exiot R., 185. 
CLAYTON, W., 97. 

CLEMENT, 72. 

CLOWES, G. H. A., 96. 
ConNELLY, JOHN, 191. 
Corton, 195. 

CorrrELt, F. H., 53, 66, 97, 107. 
CovuETTE, "54. 

CRILE, 186. 

CuNNINGHAM, E., 20. 
CusHMAN, A. S., 79. 
DALTON, 8. 

Darwin, Cuas., 176. 
Davey, W. P., 95. 

Davis, J. O., 74. 

DE Bruyn, Losry, 6, 44. 
DEMPSTER, 8. 

Dopp, H. V., 107. 

Donnan, 135. 

DRaPER, 96. 

Dusosc, 91. 

Duc aux, J., 166. 
Dunstan, 104. 

EInstTEIN, A., 7, 26. 
ExuRuicu, P., 166, 192. 
ENGLER, 54. 

Farabay, M., 21, 38, 34, 38. 
FICKENDAY, 75. 

FInK1ge, P., 96. 

FInsEen, 184. 

Fiscumr, M. H., 99, 166, 195. 
epee Propucts LABORATORIE , 
Forster, 45. 

FREUNDLICH, H., 40, 172. 
FRIEDMANN, 41. 

GARDNER, H. A., 94. 
Gastou, Paut, 195. 

GIBBS, 2 WILLARD, 13, 96. 
Gitz, PBs 7% 

GorTNER, R. A., 45. 
GRAHAM, THOMAS, 1, 24, 47, 53, 54. 


199 


200 


GrRUBLER, 87. 
Gurpakov, N., 87. 
HAEUSSLER, 146. 


Hampton, 166. 
Harpy, W. B., 27, 40, 41, 45, 51, 
53, 102, 180. 


HARKINS, W. D., 8, 96. 
HARRISON, W., 86. 
HATScHEK, E., ” 45, 195. 
HEILBRONN, Ne 172. 
HEILBRUNN, Te ‘V.. 172. 
Hxtn-SHaw, 111. 
Henna, V., 142. 
HERRERA, A. L., 174, 178. 
HitDEBRAND, lp HL, 06. 
Hitt, Crort, 170. 
HitiyEr, J., 99. 
HoFFMANN, 45. 
Hormes, H. N., 195. 
Honpa, K., 145, 152. 
Houtett, 194. 

Hutu, A. W., 146. 
HumpuHerey, 108. 
JACKSON, H., 45, 100. 
JacosI, A., 129, 130. 
JANDER, 111. 
JEFFERIES, Zay, 144. 
JUSTSCHENKO, A. yi 187. 
KENDALL, E. C., 
KEPPLER, 75. 

Kine, P. E., 86. 
KIRSCHBRAUN, L., 108. 
Kein, A. A., 163. 


K. O., 142. 


Lascary, L., 18. 
Lea, M. CAREY, 43. 

Le CuaTeier, H H., 163 
pe S., 174. 
LEHNER, v., 19, 23. 
Lz Saez, 21. 
Lewis, G. N., 
LEwIs, ISABEL iM, 57. 
LEWKOWITCH, 98. 
LIESEGANG, R. E., 50, 174, 178. 
Linu, R. 8. 175, 194. 
LIPPMANN, 44, WES 
LIvINGsTON, 74. 
LopaE, Srr OLIVER, 53, 66. 
Lor, in 135. 
LOTTERMOSER, A., 38, 41, 114. 


AUTHOR INDEX 


Lumiztre, Avaustn, 75, 187%, 
195. 


MacMicwast, 54. 

McBain, J. W., 98, 99, 119. 

Martens, 143. 

Mary, ALBERT, 168. 

MaxwELt, J. CLERK, 60. 

MENDELEEF, 19. 

Menrica, 18, 148. 

MERKLEN, 08. 

MILukaN, R. A., 8, 58. 

MIcHAELIS, LEONAR, 12. 

MIcHAELIS, W., 163. 

Molssan, 162. 

Moogrrs, 166. 

Moors, Wo., 110. 

Movton, 195. 

Mutier, ArtTHuR, 90, 161, 195. 

NEISSER, 41. 

NEVILLE, 159. 

NEwcoms, Simon, 60. 

NEWTON, fe 19, 20. 

OsTWALD, WILHELM, 54. 

OstwAL, WOLFGANG, 26, 29, 40, 
195. 

PARACELSUS, 167. 

Patrick, W., 108. 

PERRIN, J., 40, 195. 

Puiiurps, A. J., 163. 

PICKERING, S. V., 91, 144. 

Przront, A., 29. 

Puavson, H., 25. 

PowARNIN, G., 119. 

PRANGE, 91 

Pratt, Wm. R., 123. 

Procter, H. R., 119. 

Potz, 141. 

Rarro, M., 29. 

Ramsay, Sir Wo., 6. 

RANDALL, 13. 

Riwzat, E. K., 169. 

Ricuarps, T. W., 8 142. 

RICHARDSON, CLIFFORD, 108. 

Ricuarpson, W. D., 101. 

RrivibReE, 72. 

RoBeERtTs-AUSTEN, 143, 159. 

RoHLAND, P., 163. 


Ross, H., 3. 

RosenHaIn, W., 146, 155, 157, ° 
158, 159. 

RuncE, F. EB. 178. 

RUSSELL, Sir E. J., 75. 

SAUVEUR, A., 18. 

SCHADE, H., 183, 


AUTHOR INDEX 201 


ScHALEK, 173. 
ScHERRER, P., 141, 144. 
ScHREINER, 74. 
ScHUPPHAUS, 91. 
Scort, 18, 143. 
Serrriz, W., 172, 178. 
SEYEWETZ, 75. 
SHAPLEIGH, 11. 
SrepEentopr, H., 33, 35. 
SINEK, 191. 

Sorsy, 1438. 

SORENSEN, 11. 
SouTHCOMBE, 103. 
SPANGENBERG, 75. 
Spencer, H. M., 113. 
SPENCER, HERBERT, 20. 
Sprine, W., 29, 100. 
SPROXTON, F., is me 91. 
Stopss, M. C., 


SVEDBERG, T., at "35, 173, 195. 


SzEGVARY, 173. 
TAMMANN, 138. 
TraGueE, O., 41. 
THOLE, 104. 
THOMPSON, SiR J. J., 8. 
Troost, 143. 

Tutte, J. B., 123. 


VAN BEMMELEN, J., 195. 
VAN CaLcaR, 6. 
VEGARD, Lars, 58. 
vANn’T Horr, 169. 

von Mmyemr, 38. 

VON Paat, 48. 

VON WEIMANN, P. P., 26, 158, 159. 
WALTENBERG, 18, 143. 
Ware, 94. 

WarREN, L. F., 66. 
Weiser, H. J., 165. 
WELLS, 103. 

WELLS, P. E., 21. 
Wey, 127. 

WHEELER, 104. 
Wuyrmper, R., 126, 128. 
WIEGNER, 182. 
Witiows, 195. 
Witson, C. T. R., 59. 
Witson, J. A., 119. 
WISLICENUS, H., 174. 
Wyatt, B. H., 67 
Yosuimoro, J., 166. 
ZSIGMONDY, R., 5, 13, 27, 31, 33, 


35, 38, 39, 40, 48, 111, 112, 139, 
140, 195 


Zonz, E., 46, 131. 


SUBJECT INDEX 


A, B, C method, 112. 
Absorption, 189. 

Acidity, total, 12. 
Acidosis, 189. 

Activated carbon, 108. 
Activated sludge process, 113. 
Active immunity, 193. 
Addition agents, 161. 
Adhesion, 23. 

Adsorption, 22, 119. 
Aerosols, 65. 

Agar, 125. 

Ageing, 82. 

Ageing processes, 183. 
Agriculture, 73. 

Air bubbles, 26, 83. 
Albumoses, 131. 

Alcohol, solidified, 107. 
Allocolloid, 146. 

Alloys, 140. 

Alum tannage, 119. 
Ambergris, 69. 

Amethyst, 72. 

Amicron, 35. 

Amorphous theory of metals, 159. 
Amphiasters, 172. 
Analysis, ultra-filters for, 111. 
Anaphylaxis, 187, 188, 193. 
Annealing, 153. 

Antigen, 167. 

Antipyrine, 167. 
Antiseptics, 168. 
Antitoxins, 188. 
Antharyxalon, 105. 
Anthrax, 187. 

Arabic, gum, 167. 

Arabic gum, gold number, 39. 
Argyrol, 167. 

Arsenate, lead, 109. 
Arsenic sulphid, 48. 
Asphalt, 108. 

Asses’ milk, 129. 

Asthma, 193. 

Astral rays, 174. 
Astronomy, 57. 

Atom, 10. 

Atomic forces, 21. 


Atmosphere, 63. 

Atmospheric nucleation, 65. 

Aurum potabile, 37 

Austenite, 143. 

Auto-protection, 45, 141, 179. 

Baade’s comet, 60. 

Bacilli, Table V, facing 32. 

Bacteriology, 168. 

Baking, 125. 

Barometric pressure, 63. 

Bateing, 117. 

Bauxite, 71. 

Bavarian cream, 125. 

“Beauty clays,’”’ 80. 

Benedict’s solution, 166. 

Bentonite, 76, 80. 

Benzopurpurin, 88. 

Bibliography, 195. 

Bierstein, 115. 

Bile, 188. 

Bio-colloids, 180, 181. 

Bio-electric currents, 194. 

Biology, 169. 

Bismuth oxid sol, 48. 

Blanc fixe, 94. 

Blasting gelatin, 92. 

Blood, coagulation of, 168. 

Blood corpuscle, Table V, facing 32. 

Bloom, in oil, 107. 

Blotting paper, 121. 

Blue ointment, 167. 

Blunging, 82. 

Blushing, 184. 

Boiler scale, 162. 

Boulders, 70. 

Bowman’s capsule, 191. 

Brass, 156. 

Break, in oils, 92. 

Bread, 125. 

Bronze, 159. 

Brownian motion, 16, 40, 100. 

Buffer salts, 185. 

Butter, 125. 

Calor, 183. 

Camphor, 91. . 

Camphorylphenylthiosemicarba- 
zide, 45. 


202 


SUBJECT INDEX 


Candies, 134. 

Capillary circulation, 183. 
Capillary phenomenon, 78. 
Carbon, activated, 108. 
Carburizing, 154. 

Case hardening, 154. 
Casein, 48. 

Casein solution, gold number, 39. 
Casting slip, 81. 

Casts, 182. 

Catalyzers, colloidal, 170. 
Cataplasms, 80 

Cement, 162. 

Ceramics, 81. 

Celluloid, 91. 

Cellulose, 89. 

Charcoal, 193. 

Charlotte Russe, 132. 
Chemical analysis, 165. 
Chemical compound, 9, 22. 
Chemical forces, 9. 
Chemo-therapy, 192. 
Chinese cookery, 124. 
China wood oil, 92. 
Cholesterin, 169, 188. 
Chromates, 93. 
Chromatin, 172. 

Chrome yellow, 93. 

- Chrome tannage, 119. 
Chromosomes, 172. 
Clarain, 104. 

Clays, 70, 76, 79. 

Clay, colloidal, 84. 

Clay slip, 80. 

Clouds, 61. 

Coal, 104. 

Cocci, Table V, facing 32, 
Codliver oil, 97, 167. 
Cohen’s law, 50. 

Collagen, 117. 

Collargol, 167. 

Colloid chemistry, definition, 4. 
Colloid mill, 25, 120. 
Colloid theory of metals, 159. 
Colloid-therapy, 192. 
Colloidal clay, 84, 108, 121. 
Colloidal fuel, 105. 


Colloidal gold, Tables V and VI, 32. 


Colloidal mastic test, 191. 
Colloidal mercury, 167. 
Colloidal particles, primary, 10. 


Colloidal particles, secondary, 10. 


Colloidal protection, 37. 
Colloidal silver, 167. 


203 


Colloidal solution, 22. 

Colloidal solutions, classification 
of, 27. 

Colloidal state, principle under- 
lying, 16. 

Colloidal sulphur, 167. 

Colloidality, zone of maximum, 17. 

Colloids, classification of, 26. 

Comet, 59. 

Comet’s tails, 59. 

Complement, 167. 

Confectionery, 134. 

Congo red, 88. 

Convoluted tubule, 191. 

Cosmic dust, 57. 

Cottrell process, 66. 

Cows’ milk, 129. 

Cream, 125. 

Cretinism, 65. 

Crystallogens, 139. 

Cultivation, 77. 

Cumulative protection, 46, 137, 
144, 173. 

Curd, greasy, 130. 

Cutting oils, 103. 

Cyaniding, 154. 

Cylindroids, 191. 

Cytology, 171. 

Cytoplasm, 178. 

Dark field, 72. 

De-inking paper, 121. 

Deltas, 71. 

Dementia praecox, 187. 

Detergent action, 99. 

Le eee gold number, 
40. 

Developers, 114. 

Devitrification, 137. 

Dextrin, gold number, 39. 

Dextrin, 48. 

Diabase, 72. 

Diabetes, 189. 

Diagnosis, 191. 

Dialysis, 4, 47. 

Diarrhea, 189. 

Diastase, 169, 171. 

Dibenzoyl]-1-cystine, 45. 

Differential diffusion, 50, 174. 

Diffusion, 1, 10, 49. 

Diffusion in soils, 78. 

Digestion, 188. 

Dihydrol, 23. ! 

5-dimethylaminoanilino-3, 4-di- 
phenylcyclo-1, 2-dione, 45. 


204. SUBJECT INDEX 


Diphtheria toxin, 48, 188. Exudative processes, 183. 
Dirt, 54, 100. Fabrics, shower proofing, 89. 
Dispersoids, 26. ' | Faraday-Tyndall effect, 34, 59, 63. 
Diuretics, 189. Fassgelager, 115. 
Dolor, 183. Fat tannage, 120. 
Donnan theory, 135. Fats, 188. 
Dopplerite, 105. Feathers, 64. 
Double protection, 43. Fehling’s solution, 166. 
Drenching, 117. Feldspar, 79. 
Durain, 104. Ferric oxid sol, 48. 
Duralumin, 18. Ferrite, 148. 
Dust, cosmic, 57. Fillers in rubber, 122. 
Dusts, 65, 80. Filter paper, 121. 
Dyeing, 18, 23, 86. Filtration, 111. 
Earth, 70. Fining beer, 116. 
Earthquakes, 71. Fire balls, 57. 
Eau-de-vie de Danzig, 167. Fire clay, 82. 
Edema, 182. Fire foam, 109. 
Effective acidity, 170. Fixateur, 105, 
Effective reaction, 12. ; Flint, 71. 
Egg albumin, gold number, 39. Flocculates, 187. 
Egg white, gold number, 39. Flotation, 82. 
Electric charge of colloidal parti- | Flour, 126. 

cles, 50. Fogs, 61, 65. 
Electric charge of insecticides, 110. | Foods, 124. 
Electric fish, 62. Forces, atomic, 21. 
Electrical double layer, 51. Forces, electronic, 21. 
Electro-deposition, 161. Forces, molar, 21. 
Electron, 8, 10. Forces, molecular, 21. 
Electronic forces, 21. Formol test, 191. 
Electrophoresis, 50. Fossils, 71. 
Elements, divisibility of, 7. Free water, 77. 
Emerald, 72. French varnish, 93. 
Emulsoids, 27. Frost fogs, 62. 
Emulsion, grainless, 44. Frost, prevention of, 67. 
Emulsions, 95. Froth, 838. 
Emulsions, breaking of, 97. Fuel, colloidal, 105. 
Emulsions, colors of, 115. Fusain, 104. 
Emulsions, oil, 103. Galaxy, 11. 
Emulsions, photographie, 113. Gangue, 82. 
Emulsostatic action, 46. Gas surfaces, 116. 
Endocrines, 184. Gastric juice, 190. 
End-piece, 167. Gel formation in cells, 173. 
Engine sizing, 120. Gelatin, 114, 116, 180. 
Enzymes, 169, 188. Gelatin, gold number, 39. 
Eozoon, 71. Gelatin sol, 48. 
Epilepsy, 187. Gems, 72. 
Eutectic, 150. Geology, 70. 
Kutectic drop, 82, 100. Germ plasm, 176, 180. 
Eutectoid steel, 148. Glanz coal, 105. 
Evolution, 175. Glass, 187. 
Excretion, 189. Glaucoma, 182. 
Explosives, 92. Gliadin, 126. 


Exudate, 188. Globulin, 126. 


SUBJECT INDEX 


Glomerular structure, 191. 
Glucose, 134. 

Glue, 184. 

Glue, gold number, 39. 
Gluten, 126. 

Glutenin, 126. 


Glycoprotein, gold number, 39. 


Goiter, 65. 


Gold, colloidal, Tables V and VI, 32. 


Gold number, 39. 

Gold sols, 39, 48. 
Gomorrah, 66. 

Grain, in emulsions, 115. 
Grainless emulsion, 44. 
Gravel, 70. 

Gravitation, 19, 20. 
Gravitational water, 77. 
Greases, lubricating, 103. 
Greasy curd, 130. 
Grinding, 24. 

Grog, 81. 

Growth, 174. 

Griibler’s stains, 87. 
Gum arabic, 130, 167. 
Gum drops, 134. 

Gums, gold numbers of, 39. 
Gymnotus, 62. 
H-hyperionia, 183. 
Haeussler alloys, 146. 
Halos, 62. 

Hardening, 154. 
Hardenite, 148. 

Hay fever, 64. 

Healing, 188. 
Hemoglobin, 48. 
Heterogeneity, 13. 
High speed steel, 145. 
Hollandaise sauce, 125. 
Homogeneity, 13. 
Homogenized milk, 132. 
Hormonal control, 184. 
Humic acid, 75. 
Humors, 192. 

Humus, 74. 


Hydrogen ion concentration, 11. 


Hydrolyzable salts, 190. 
Hydrophile, 40. 
Hydrophile colloids, 27. 
Hydrophobe, 40. 
Hydrophobe colloids, 27. 
Hyperglycemia, 182, 189. 
Hyperplethoria, 183. 
Hypersensitization, 188. 
Hyperthermia, 183. 


Tatrochemists, 167. 
Ice, colloidal, 26, 62. 
Ice cream, 13838. 
Ichthyol, 167. 
Immunity, 187, 193. 
Impurities, 161. 
Inflammation, 183. 
Insanity, 187. 
Insecticides, 109. 
Interfaces, 14. 
Jodine, 64. 

Iron, 142. 
Irreversible colloids, 27. 
Isinglass, 116. 


Isinglass, gold number, 39. 


Isocolloidism, 141. 
Isoelectric point, 113. 
Isotopes, 8. 

Kaolin, 72, 192. 
Kaolinite, 72. 
Karokinesis, 172. 
Kidney, 182, 190, 191. 
Kinetic motion, 16. 


Koch-Procter method, 119. 


Koch’s bacillus, 168. 
Krakatoa, 64. 
Lactose, 128. 

Latent image, 114. 
Latex, 121, 122. 
Leather, 117. 
Leather, artificial, 90. 
Liesegang’s rings, 50, 74. 
Lightning, 62. 
Linseed oil, 92. 
Lipase, 169. 


205 


Lippmann’s grainless emulsion, 44. 


Lithopone, 94 

Litmus, 48. 

Livering, 94. 

Loam, 70. 

Loneal, 153. 

Loose skein, 172. 

Low carbon steel, 147. 
Lubrication, 102. 
Lyophile, 40. 
Lyophile colloids, 27. 
Lyophobe, 40. 
Lyophobe colloids, 27. 
Lysalbinic acid, 48. 
Maasen fillers, 112. 
Magnesia, milk of, 167. 
Magnetism, 146. 
Malpighian tufts, 190. 


Maniac-depressive insanities, 187, 


206 SUBJECT INDEX 


Martensite, 143. 
Marshmallows, 134. 
Mat coal, 105. 
Mayonnaise, 124. 
Meats, 124. 
Medicine, 169. 
Meerschaum, 26. 
Metals, 139. 
Metaphase, 173. 
Metastasis, 3. 
Meteors, 57. 
Meteorology, 61. 
Micelle, 99. 
Middle-piece, 167. 
Migration, 50. 
Military use of colloids, 68. 
Milk, 48, 128. 


Milk, rennin coagulation of, 46. 


Mill, colloid, 25. 

Mineral charcoal, 104. 
Mineralogy, 71. 

Mitosis, 172. 

Molar forces, 21. 
Molecular association, 22. 


Molecular attraction, radius of, 29. 


Molecular forces, 21. 
Molecular groups, 10. 
Molecules, 10, 
Moon-dogs, 62. 
Mordant, 87. 
Mortar, 162. 

Mother of coal, 104. 
Mucin, 188. 

Muntz metal, 157. 
Myelins, 180. 
Nebulae, 58, 61. 
Nephritis, 182. 
Neurosis, 186. 

Nigre, 98. 
Nitrocellulose, 18, 89. 
Normalizing, 154. 
Odors, 69. 

Oil, 84. 

Oil sands, 108. 

Oil wells, 108. 

Oils, cutting, 103. 
Onion skin paper, 120. 
Opal, 71, 72. 


Ophthalmia neonatorum, 167. 


Ore, 84. 

Orientation in surfaces, 177. 
Osmotic hyperionia, 183. 
Ovomucoid, gold number, 39. 
Oysters, 125. 


Oxidation processes, 181. 


Pan, 79. 

Pancreatic juice, 188, 190. 
Pancreatin, 117, 169. 
Papain, 169. 

Paper, 120. 

Passive immunity, 193. 
Patent leather, 90. 
Pathological oxidation, 182. 
Patriotic test tube, 49. 
Pearl, 72. 

Pearlite, 143. 

Pebbles, 70. 

Pectization, 52. 

Pedesis, 100. 

Pedetic motion, 6. 
Pepsin, 169, 171, 188. 
Peptization, 52. 
Perfumes, 68. 

Perihelions, 62. 
Periselenes, 62. 
Petroleum, 107. 
Pharmacy, 167. 

Phase rule, 13. 

Phases, 13. 

Photography, 113. 
Photo-halides, 114. 
Physical forces, 9. 
Physical mixture, 9. 
Pigments, 92. 

Pitch, 108. 

Plants, 193. 

Plaster, 162. 

Plaster of Paris, 78, 164. 
Platinum sol, 48. 
Ploughing, 76. 

Plural protection, 43, 113. 
Pogonip, 63. 

Poisons, insect, 109. 
Polymerization, 22. 
Polyuria, 189. 
Potassium bromate, 128. 
Pottery, 82. 

Precipitin test, 166. 
Prince Rupert’s drops, 137. 
Proliferating processes, 183. 
Protalbumose, 48. 
Protection, colloidal, 37. 


Protection, cumulative, 46, 137. 


Protection, double, 43. 
Protection, plural, 43, 113. 
Protective colloids, 74, 188: 


SUBJECT INDEX 


Protective colloids in milk, 129, 
130 


Proton, 10. 

Protoplasm, viscosity of, 172. 

Prussian blue, 48, 

Psychiatry, 186. 

Psychosis, 186. 

Ptyalin, 188. 

Puering, 117. 

Pugging, 82. 

Pukall filters, 112. 

Pulverizing, 24. 

Pumice, 26. 

Punctum vegetationis, 174. 

Purple of Cassius, 31, 37, 38, 140. 

Putty, 144. 

Puttying, 94. 

Quasi-crystals, 177. 

Radius of molecular attraction, 
29. 

Rain, 67. 

Reaction, effective, 12. 

Recalescence, 148. 

Refractories, 81. 

Relativity, 7. 

Rennin coagulation of milk, 132. 

Re-solation, 173. 

Reversible colloids,-27. 

Ripening of emulsions, 114. 

Root pull, 77. 

Rosin size, 120. 

Rouget cells, 184. 

Rubber, 122, 

Rubber, dispersion of, 123. 

Rubber latex, 121, 122. 

Rubor, 183. 

Ruby, 72. 

Ruby glass, 138, 139. 

Salvarsan, 192. 

Sand, 70. 

Scallops, 125. 

Scarlet fever, 182. 

Secretion, 189. 

Selenium oxychloride, 22. 

Senility, 187. 

Septic tank, 112. 

Serpentine, 72. 

Serum albumin, 48. 

Sewage disposal, 112. 

Shellac, 93. 

Shock, 187. 

Shortening, 128. 

Shower-proofing, 89. 

Shrinking, 189. 


207 


Siezing, 102, 108. 

Silica gel, 108. 

Silicic acid, 1, 53, 54. 

Silicic acid sol, 48. 

Silver sols, 38. 

Skinning, 94. 

Sky, color of, 63. 

Smoke screens, 68. 

Smokes, 65. 

Smudge pots, 67. 

Snow, 64 

Soaps, 19, 98. 

Soaps, transparent, 101. 
Sodium oleate, gold number, 40. 
Sodium stearate, gold number, 40. 
Sodom, 66. 

Soil water, 76. 

Soils, 76. 

Solder, 156. 

Solidified alcohol, 107. 


‘ Solution, 5, 22. 


Solution, colloidal, 22. 

Staling of bread, 126. 

Somatoplasm, 181. 

Sorbite, 143. 

Sorption, 119. 

Specific action, 169. 

Sphero-crystals, 179. 

Spiral nebulae, 58. 

Spireme, 172. 

Spirillae, 187. 

Spirochete pallidum, 187. 

Spheroidizing, 154. 

Spherulitic form, 138. 

Spontaneous combustion, 104. 

Stannic acid, gold number, 40. 

Starch, gold number, 39. 

Steam, 61. 

Steel, 18, 142, 147. 

Steel, high speed, 145. 

Stepped transformation in steel, 
152. 

Stream-line filter, 111. 

Styptics, 168. 

Submicron, 35. 

Sugar, gold number, 40. 


Sulfur, 18, 161. 


Sulfur, colloidal, 29. 
Sun-dogs, 62. 

Surface forces, 24. 

Surface, free, 10. 

Surface tension, 82, 89, 195. 
Suspension, 5. 

Suspensoids, 27. 


208 


Swelling, 181, 189. 

Swelling of tissue, 185. 

Symmetrical orientation, 176. 

Syneresis, 119. 

Takoma soil, 74. 

Tanning, 23, 117. 

Tempering, 148, 154. 

Tetanus toxin, 188. 

Therapeutics, 167. 

Therapy, 192. 

Thyroxin, 64. 

Tight skein, 172. 

Tillage, 78. 

Tilth, 75. 

Time factor, 142. 

Tin disease, 142. 

Tin-lead alloys, 155. 

Tin tree, 161. 

Toad-skin dintment, 192. 

Topaz, 72. 

Toxin, diphtheria, 48, 188. 

Toxin, tetanus, 188. 

Tragacanth, 167. 

Tragacanth gum, gold number, 39. 

Transparent soaps, 101. 

Transudate, 188. 

Troosite, 1438. 

Trypsin, 188. 

Tub sizing, 120. 

Tuberculosis, 168. 

Tumor, 183. 

Turgidity, 184. 

Twilight, 63. 

Tyndall effect, 34, 59, 91. 

Type metal, 156. 

Unfree water, 77. 

Unleavened bread, 124. 

Ultra-clay, 73. 

Ultra filters, 111. 

Ultrafiltration, 47. 

Ultramicron, 35. 

Ultramicroscope, 10, 33, 81. 

Ultramicroscopic examination of 
casts, 191. 

Ultramicroscopic examination of 
enzymes, 170 

Ultramicroscopic examination of 
metal, 162. 


SUBJECT INDEX 


Ultramicroscopic examination of 
milk, 131. 

Ultramicroscopic examination of 
oil, 104 

Ultramicroscopic examination of 
toxins and antitoxins, 188. 

Ultramicroscopic observations on 
soap, 99, 102. 

Urea, gold number, 40. 

Urine, 191. 

Vacuum fermentation process, 116. 

Varnishes, 92. 

Vas afferens, 191. 

Vas defferens, 191. 

Vaso-dilation, 190. 

Vegetable tannage, 118. 

Viscose, 18. 

Viscosity, 54. 

Viscosity of oils, 104. 

Viscosity of soap solutions, 98. 

Vital force, 169. 

Vitrain, 104. 

Volcanic dust, 64. 

Volcanos, 64. 

Vollmiindigkeit of beer, 116. 

Vulcanization, 122. 

War, colloids in, 68. 

Warping, 81. 

Wassermann test, 166, 191. 

Water japan, 95. 

Water in soils, 77. 

Water, colloidal, 26. 

Weather, influence of, 63. 

Weathering, 70. 

Whipped cream, 132. 

Widal test, 168, 191. 

Wilting coefficient, 77. 

Wort, 115. 

Wounds, 188. 

X-rays, 72. 

X-ray spectrometer, 10. 

Yeast food, 128. 

Zeppelins, 68. 

Zodiacal light, 58. 

Zinc-copper alloys, 156. 

Zone of maximum colloidality, 
87, 90, 135. 

Zones of precipitation, 41. 








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